Herb Butter Portobello Burger With Swiss and Caramelized Onions

Imperial May 31, 2026May 31, 2026

Quick Recipe Version (TL;DR)

Yield: 4 burgers
Prep Time: 20 minutes
Cook Time: 45 minutes
Total Time: 1 hour 5 minutes

Quick Ingredients

Portobellos: 4 large portobello mushroom caps, 3 tablespoons olive oil, 2 tablespoons balsamic vinegar, 1 tablespoon low-sodium soy sauce, 1 teaspoon Dijon mustard, 1/2 teaspoon smoked paprika, 1/2 teaspoon kosher salt, 1/2 teaspoon black pepper
Garlic herb butter: 4 tablespoons softened unsalted butter, 2 finely grated garlic cloves, 1 tablespoon chopped parsley, 1 teaspoon chopped chives, 1 teaspoon thyme leaves, 1/2 teaspoon lemon zest, 1/4 teaspoon kosher salt, 1/4 teaspoon black pepper
Caramelized onions: 2 large yellow onions, 1 tablespoon olive oil, 1 tablespoon unsalted butter, 1/2 teaspoon kosher salt, 1 teaspoon balsamic vinegar, 2 tablespoons water as needed
Assembly: 4 brioche or bakery burger buns, 4 slices Swiss cheese, 2 cups baby arugula, 1/4 cup mayonnaise, 1 tablespoon Dijon mustard, 1 teaspoon balsamic vinegar

Do This

1. Cook sliced onions with olive oil, butter, and salt over medium to medium-low heat for 30 to 35 minutes, adding a splash of water if they stick; finish with 1 teaspoon balsamic vinegar.
2. Mix softened butter with garlic, parsley, chives, thyme, lemon zest, salt, and pepper.
3. Whisk the mushroom marinade, brush it over the portobello caps, and let them sit for 10 minutes while the grill heats to 425°F to 450°F.
4. Grill mushrooms for 4 to 5 minutes per side until tender and lightly charred.
5. Place herb butter and Swiss cheese on each mushroom, cover the grill, and cook 1 to 2 minutes until melted.
6. Toast buns for 1 to 2 minutes and stir together the mayonnaise, Dijon, and balsamic spread.
7. Build each burger with spread, arugula, a cheesy portobello cap, caramelized onions, and the top bun.

Why You’ll Love This Recipe

Steakhouse flavor without the steak: A smoky grilled portobello, garlic herb butter, melted Swiss, and sweet onions make every bite rich and savory.
Truly satisfying: Portobello caps have a naturally meaty texture, so this vegetarian burger feels hearty rather than like a side dish on a bun.
Simple ingredients, big payoff: Butter, garlic, herbs, balsamic, and onions do most of the heavy lifting.
Great for cookouts: These grill quickly once the onions are ready, and they are easy to serve alongside classic burger toppings and fries.

Grocery List

Produce: 4 large portobello mushroom caps, 2 large yellow onions, garlic, fresh parsley, fresh chives, fresh thyme, 1 lemon, baby arugula
Dairy: Unsalted butter, Swiss cheese slices
Pantry: Olive oil, balsamic vinegar, low-sodium soy sauce, Dijon mustard, mayonnaise, smoked paprika, kosher salt, black pepper, 4 brioche or bakery burger buns

Full Ingredients

For the Caramelized Onions

2 large yellow onions, thinly sliced from root to tip, about 1 pound total
1 tablespoon olive oil
1 tablespoon unsalted butter
1/2 teaspoon kosher salt, preferably Diamond Crystal; if using fine sea salt, use 1/4 teaspoon
1 teaspoon balsamic vinegar
2 tablespoons water, as needed to prevent scorching

For the Garlic Herb Butter

4 tablespoons unsalted butter, softened to room temperature
2 garlic cloves, finely grated or minced into a paste
1 tablespoon finely chopped fresh parsley
1 teaspoon finely chopped fresh chives
1 teaspoon fresh thyme leaves, lightly chopped if the leaves are large
1/2 teaspoon finely grated lemon zest
1/4 teaspoon kosher salt, preferably Diamond Crystal
1/4 teaspoon freshly ground black pepper

For the Grilled Portobello Caps

4 large portobello mushroom caps, 4 to 5 inches wide, stems removed
3 tablespoons olive oil
2 tablespoons balsamic vinegar
1 tablespoon low-sodium soy sauce
1 teaspoon Dijon mustard
1/2 teaspoon smoked paprika
1/2 teaspoon kosher salt, preferably Diamond Crystal; if using fine sea salt, use 1/4 teaspoon
1/2 teaspoon freshly ground black pepper

For the Steakhouse Dijon Spread

1/4 cup mayonnaise
1 tablespoon Dijon mustard
1 teaspoon balsamic vinegar
1/8 teaspoon freshly ground black pepper

For Assembly

4 brioche or bakery-style burger buns, split
4 slices Swiss cheese, about 1 ounce each
2 cups baby arugula, loosely packed
Caramelized onions, from above
Grilled portobello caps with garlic herb butter and Swiss, from above
Steakhouse Dijon spread, from above

Herb Butter Portobello Burger With Swiss and Caramelized Onions – Closeup

Step-by-Step Instructions

Step 1: Caramelize the onions

Place a large 12-inch skillet over medium heat. Add 1 tablespoon olive oil and 1 tablespoon unsalted butter. When the butter has melted, add the sliced onions and 1/2 teaspoon kosher salt. Stir well so the onions are lightly coated.

Cook for 8 minutes, stirring every couple of minutes, until the onions soften and begin to release moisture. Reduce the heat to medium-low and continue cooking for 22 to 27 minutes, stirring often, until the onions are deep golden, soft, and jammy. If browned bits start sticking aggressively to the pan, add 1 tablespoon water and scrape them up with a wooden spoon. Repeat once more if needed.

Stir in 1 teaspoon balsamic vinegar during the final 2 minutes of cooking. Remove the onions from the heat and set them aside. You should have about 1 cup of caramelized onions.

Step 2: Mix the garlic herb butter

In a small bowl, combine 4 tablespoons softened unsalted butter, 2 finely grated garlic cloves, 1 tablespoon chopped parsley, 1 teaspoon chopped chives, 1 teaspoon thyme leaves, 1/2 teaspoon lemon zest, 1/4 teaspoon kosher salt, and 1/4 teaspoon black pepper.

Mash everything together with a fork until evenly combined. The butter should be soft and spreadable, with the herbs and garlic distributed throughout. Set it aside at room temperature while you grill the mushrooms.

Step 3: Prepare and marinate the portobello caps

Use a damp paper towel to wipe the portobello caps clean. Do not rinse them under running water, because mushrooms absorb water easily and can become soggy. Remove the stems if they are still attached. If the gills are very dark or sandy, gently scrape them out with a spoon; this is optional, but it helps keep the burger from becoming too dark and juicy.

In a shallow dish, whisk together 3 tablespoons olive oil, 2 tablespoons balsamic vinegar, 1 tablespoon low-sodium soy sauce, 1 teaspoon Dijon mustard, 1/2 teaspoon smoked paprika, 1/2 teaspoon kosher salt, and 1/2 teaspoon black pepper. Brush the mixture all over the mushrooms, including the rounded tops and the gill sides. Let the mushrooms sit for 10 minutes while the grill preheats.

Step 4: Preheat the grill

Preheat an outdoor grill to medium-high heat, about 425°F to 450°F. Clean and oil the grates. If using a grill pan indoors, heat it over medium-high heat for 4 to 5 minutes until hot, then lightly oil the ridges.

The grill should be hot enough to give the portobellos light char marks without burning the marinade. If your grill runs very hot, aim for the cooler side of medium-high and keep the lid open during the first few minutes.

Step 5: Grill the portobellos and melt the Swiss

Place the mushrooms on the grill gill-side down first. Grill for 4 to 5 minutes, until the edges begin to soften and the underside has grill marks. Flip the mushrooms and grill the rounded side for another 4 to 5 minutes, until the caps are tender but not collapsed.

Turn the mushrooms gill-side up. Spoon about 1 tablespoon garlic herb butter into the center of each cap, then place 1 slice of Swiss cheese over the butter. Close the grill lid and cook for 1 to 2 minutes, just until the cheese melts and the butter softens into the mushroom. Transfer the mushrooms to a plate and let them rest for 3 minutes so the juices settle slightly.

Step 6: Toast the buns and make the spread

Place the split buns cut-side down on the grill for 1 to 2 minutes, until lightly toasted. Watch them closely, especially if using brioche buns, because they can brown quickly.

In a small bowl, stir together 1/4 cup mayonnaise, 1 tablespoon Dijon mustard, 1 teaspoon balsamic vinegar, and 1/8 teaspoon black pepper. This quick spread adds tang and a steakhouse-style richness that balances the sweet onions and buttery mushroom.

Step 7: Assemble the burgers

Spread the Dijon mixture on the cut sides of the toasted buns. Add a small handful of arugula to each bottom bun. Place one grilled portobello cap on top of the arugula, cheese-side up. Spoon a generous amount of caramelized onions over the melted Swiss.

Close with the top buns and serve right away while the mushrooms are warm, the cheese is melted, and the arugula still has a little peppery bite.

Pro Tips

Do not soak the mushrooms: Wiping them clean keeps the texture meaty instead of watery.
Scrape the gills if you prefer a cleaner bite: The gills are edible, but removing them helps reduce dark juices and makes room for the herb butter.
Give the onions time: True caramelized onions need steady, medium-low heat. If they brown too fast, lower the heat and add a splash of water.
Let the mushrooms rest before assembling: Three minutes on a plate helps keep the burger from flooding the bun with juices.
Build with arugula on the bottom: The greens create a small barrier between the juicy mushroom and the bottom bun.

Variations

Blue cheese steakhouse version: Swap the Swiss for 3 ounces crumbled blue cheese, dividing it among the 4 mushrooms after grilling. Cover for 1 minute to soften.
Vegan portobello burger: Use plant-based butter, vegan Swiss-style slices, vegan mayonnaise, and buns made without dairy or eggs.
Spicy peppery version: Add 1/4 teaspoon crushed red pepper flakes to the herb butter and use pepper jack cheese instead of Swiss.

Storage & Make-Ahead

These burgers are best assembled right before serving, but the components are very make-ahead friendly. Store cooked portobello caps in an airtight container in the refrigerator for up to 3 days, caramelized onions for up to 4 days, and garlic herb butter for up to 1 week. The herb butter can also be frozen for up to 3 months.

To reheat the mushrooms, warm them in a covered skillet over medium-low heat for 4 to 6 minutes, or in a 350°F oven for 8 to 10 minutes. Add the Swiss cheese during the last 1 to 2 minutes of reheating. Reheat onions in a small skillet over low heat with 1 teaspoon water until warm. For the best texture, toast fresh buns and add arugula only after reheating.

If prepping ahead for a cookout, caramelize the onions up to 2 days in advance, mix the herb butter up to 5 days in advance, and clean the mushrooms up to 24 hours in advance. Wait to marinate the mushrooms until 10 to 30 minutes before grilling so they stay firm and meaty.

Nutrition (per serving)