Quick Recipe Version (TL;DR)
Quick Ingredients
- 4 onion bagels, split
- 1 lb thin-sliced roast beef
- 2 cups (about 2 oz) baby spinach, loosely packed
- 8 oz cremini or button mushrooms, sliced
- 1 small shallot (or 1/4 small onion), minced
- 1 tbsp olive oil + 1 tbsp butter (for mushrooms)
- 1/2 cup sour cream
- 2 tbsp prepared horseradish, drained
- 1 tbsp mayonnaise
- 1 tsp Dijon mustard
- Salt, black pepper, and optional 1 tsp Worcestershire sauce
Do This
- 1. Make horseradish spread: mix sour cream, horseradish, mayo, Dijon, salt, and pepper; chill.
- 2. Sauté mushrooms and shallot in olive oil and butter over medium-high heat until browned, 6–8 minutes; season and set aside.
- 3. Warm roast beef gently in a covered skillet over low heat with 2–3 tbsp water or broth until just heated.
- 4. Toast onion bagels until golden, either in a toaster or cut-side down in a dry skillet.
- 5. Spread horseradish mixture generously on both cut sides of each bagel.
- 6. Layer baby spinach, warm roast beef, and sautéed mushrooms on the bottom halves; close with tops.
- 7. Slice in half if desired and serve immediately while warm.
Why You’ll Love This Recipe
- Hearty and satisfying: tender roast beef, earthy mushrooms, and a chewy onion bagel make a full-meal sandwich.
- Big flavor with simple ingredients: creamy horseradish adds a restaurant-level punch with just a few pantry staples.
- Flexible: use deli roast beef or leftovers from a roast, and swap greens or cheese to suit your taste.
- Great for lunches, casual dinners, or entertaining; easy to scale up for a crowd.
Grocery List
- Produce: Baby spinach, cremini or button mushrooms, shallot or small onion, garlic (optional), lemon (optional), fresh herbs such as chives or thyme (optional)
- Dairy: Sour cream, butter, mayonnaise, optional sliced cheese (provolone, Swiss, or cheddar)
- Pantry: Onion bagels, thin-sliced roast beef, prepared horseradish, Dijon mustard, olive oil, salt, black pepper, Worcestershire sauce (optional)
Full Ingredients
Creamy Horseradish Spread
- 1/2 cup sour cream
- 2 tbsp prepared horseradish, well drained (more or less to taste)
- 1 tbsp mayonnaise
- 1 tsp Dijon mustard
- 1 tsp fresh lemon juice (optional, for brightness)
- 1/4 tsp kosher salt (or 1/8 tsp fine sea salt), plus more to taste
- 1/8 tsp freshly ground black pepper
- 1 tsp finely chopped fresh chives (optional)
Sautéed Mushrooms
- 1 tbsp olive oil
- 1 tbsp unsalted butter
- 8 oz cremini or button mushrooms, cleaned and thinly sliced
- 1 small shallot (or 1/4 small yellow onion), finely minced
- 1 small garlic clove, minced (optional)
- 1/4 tsp kosher salt (or to taste)
- 1/8 tsp freshly ground black pepper
- 1 tsp Worcestershire sauce (optional, for savory depth)
- 1 tsp fresh thyme leaves, or a pinch of dried thyme (optional)
Sandwich Assembly
- 4 onion bagels, split horizontally
- 1 lb thin-sliced roast beef (deli-style or leftover roast, sliced very thin)
- 2 cups loosely packed baby spinach (about 2 oz), rinsed and dried
- 2–3 tbsp water or low-sodium beef broth (for gently warming the beef)
- 4 slices provolone, Swiss, or sharp cheddar cheese (optional, for a melty version)
- Freshly ground black pepper, to finish (optional)
Step-by-Step Instructions
Step 1: Make the creamy horseradish spread
In a small mixing bowl, combine the sour cream, prepared horseradish, mayonnaise, Dijon mustard, lemon juice (if using), salt, pepper, and chopped chives. Stir until completely smooth and well blended. Taste and adjust seasoning: add more horseradish for extra heat, more salt for savoriness, or a squeeze more lemon for brightness.
Cover and refrigerate while you prepare the rest of the components. Chilling for at least 10 minutes helps the flavors meld and the sauce thicken slightly.
Step 2: Prep the vegetables
Rinse the baby spinach and dry it thoroughly using a salad spinner or by patting it with clean kitchen towels; excess moisture can make the bagels soggy. Clean the mushrooms with a damp paper towel or cloth (avoid soaking them in water), then thinly slice them. Finely mince the shallot (or onion) and the garlic, if using.
Have all your ingredients ready near the stove so that assembly later is quick and the sandwiches stay warm.
Step 3: Sauté the mushrooms for savory depth
Heat the olive oil and butter together in a large skillet over medium-high heat until the butter just begins to foam. Add the sliced mushrooms in an even layer. Let them cook undisturbed for 2–3 minutes so they start to brown. Then stir and continue cooking for another 3–4 minutes, until the mushrooms are golden in spots and have released most of their moisture.
Add the minced shallot and garlic (if using), along with the salt, pepper, thyme, and Worcestershire sauce. Cook for 1–2 minutes more, stirring frequently, until the shallot is softened and everything smells deeply savory. Taste and adjust seasoning. Transfer the mushrooms to a bowl and keep warm (cover loosely with foil).
Step 4: Warm the roast beef gently
Reduce the heat under the skillet to low. Add 2–3 tbsp water or beef broth to the pan to create a little steam. Loosely pile the sliced roast beef into the skillet in a couple of small stacks, then cover with a lid. Warm for 2–3 minutes, just until the meat is heated through and pliable. Avoid overcooking; you are only reheating, not searing, to keep the beef tender and juicy.
Once warmed, turn off the heat but keep the beef covered so it stays warm while you toast the bagels. If you are adding cheese, you can lay a slice over each pile of beef just before turning off the heat, covering again to melt slightly.
Step 5: Toast the onion bagels
While the beef is warming, toast the onion bagels. You can use a toaster or a hot, dry skillet. For a skillet method, place the split bagels cut-side down in a preheated skillet over medium heat. Toast for 2–4 minutes, until the cut sides are nicely golden and crisp. Toasting is important; it adds flavor and keeps the bagel from going soggy once the creamy spread and warm fillings are added.
Transfer the toasted bagels to a clean work surface or cutting board, cut-side up, ready for assembly.
Step 6: Assemble the sandwiches
Spread a generous layer (about 1–1½ tbsp) of the creamy horseradish mixture on both cut sides of each toasted onion bagel. On the bottom half of each bagel, arrange a layer of baby spinach leaves, covering the surface evenly.
Top the spinach with a warm stack of roast beef (and melted cheese, if using). Spoon a quarter of the sautéed mushrooms over each sandwich, making sure some of the browned bits and shallots are included. Finish with a light grind of black pepper if you like. Place the top half of the bagel over the filling and press gently to help everything hold together.
Step 7: Serve and enjoy
Serve the roast beef and horseradish onion bagel sandwiches immediately while warm, either whole or sliced in half with a sharp serrated knife. Pair with a simple green salad, kettle chips, or a cup of soup for a complete meal.
If you are making these for a crowd, you can assemble them on a baking sheet, cover loosely with foil, and keep warm in a 250°F (120°C) oven for up to 20 minutes before serving. Just be aware that the bagels will soften slightly as they sit.
Pro Tips
- Drain the horseradish. Squeeze or press prepared horseradish with a spoon to remove excess liquid before mixing, so the spread stays thick and does not make the bagels soggy.
- Do not overcrowd the mushrooms. Give them room in the pan so they brown instead of steaming. Cook in two batches if your skillet is small.
- Warm, do not cook, the roast beef. Gentle heat plus a splash of water or broth keeps the meat tender and juicy; high heat can make it tough.
- Toast the bagels well. A deep golden toast adds flavor and creates a barrier for the creamy spread and mushroom juices.
- Assemble just before serving. For the best texture and temperature contrast, do not add the spread or spinach until you are ready to eat.
Variations
- Cheesy melt version: After warming the roast beef in the skillet, top each portion with a slice of provolone, Swiss, or cheddar. Cover until just melted, then transfer to the bagel.
- Extra onion lover’s version: Add a layer of quick-pickled red onions or caramelized onions along with the mushrooms for even more onion flavor to match the onion bagel.
- Lighter twist: Use whole-wheat bagels, swap half of the sour cream for Greek yogurt, and use extra spinach plus a few slices of fresh tomato.
Storage & Make-Ahead
For best results, assemble these sandwiches right before serving. However, you can easily prep components ahead:
The creamy horseradish spread can be made up to 4–5 days in advance and stored in an airtight container in the refrigerator. The sautéed mushrooms will keep for up to 3 days, refrigerated and tightly covered; reheat gently in a skillet over low heat before using. Roast beef should be kept cold and only warmed just before assembly. Toasted bagels are best fresh, but you can slice them ahead and store them in a sealed bag for 1 day, then toast when ready to serve.
If you have leftover assembled sandwiches, wrap them tightly in foil and refrigerate for up to 1 day. Reheat in a 300°F (150°C) oven, still wrapped, for 10–15 minutes. The texture of spinach and bagel will soften, but the flavor will still be delicious.
Nutrition (per serving)
Approximate values for one fully loaded sandwich (including spread and mushrooms, without cheese): about 620 calories; 38 g protein; 55 g carbohydrates; 26 g fat; 9 g saturated fat; 4 g fiber; 7 g sugar; 1,450 mg sodium. Using cheese, extra spread, or different bagel sizes will change these numbers.
