Quick Recipe Version (TL;DR)
Quick Ingredients
- 2 large yellow onions, thinly sliced
- 2 tbsp unsalted butter + 1 tbsp olive oil
- 1/2 tsp fine salt + 1/2 tsp sugar
- 1/2 cup mayonnaise, 2 tbsp chopped parsley, 1 tbsp chopped chives
- 1 tsp Dijon mustard, 1 tsp lemon juice, 1 small garlic clove
- 2 1/2 cups shredded cooked roast chicken
- 4 everything bagels, split
- 2 cups baby spinach
- Optional: 4 slices provolone or Swiss cheese
Do This
- 1. Caramelize onions: Cook sliced onions with butter, oil, salt, and sugar over medium-low heat, stirring often, for 25–30 minutes until deep golden and jammy.
- 2. Make herb mayo: Stir together mayonnaise, parsley, chives, Dijon, lemon juice, garlic, salt, and pepper; chill until needed.
- 3. Prep chicken: Shred cooked roast chicken into bite-sized pieces; season lightly with salt and pepper if needed.
- 4. Heat oven to 375°F (190°C). Arrange split bagels cut-side up on a baking sheet.
- 5. Top bagel bottoms with chicken (and cheese if using). Toast in the oven 6–8 minutes until bagels are lightly crisp and cheese is melted.
- 6. Spread bagel tops with herb mayo. Layer spinach and a generous scoop of caramelized onions over the warm chicken. Close sandwiches.
- 7. Serve immediately while hot, with extra herb mayo and onions on the side if you like.
Why You’ll Love This Recipe
- Elevates leftover or store-bought roast chicken into a cozy, restaurant-worthy sandwich.
- Slow-cooked caramelized onions bring deep sweetness that pairs perfectly with savory chicken and tangy herb mayo.
- Everything bagels add built-in seasoning and crunch, so every bite is packed with flavor.
- Flexible and forgiving: great for meal prep, weeknight dinners, or an impressive brunch centerpiece.
Grocery List
- Produce: Yellow onions, baby spinach, fresh parsley, fresh chives (or green onions), garlic, lemon.
- Dairy: Unsalted butter, provolone or Swiss cheese (optional).
- Pantry: Everything bagels, cooked roast chicken (rotisserie or homemade), olive oil, mayonnaise, Dijon mustard, sugar, balsamic vinegar (optional), fine sea salt, black pepper.
Full Ingredients
For the Caramelized Onions
- 2 large yellow onions (about 1 lb / 450 g), peeled and thinly sliced pole-to-pole
- 2 tbsp unsalted butter
- 1 tbsp olive oil
- 1/2 tsp fine sea salt
- 1/2 tsp granulated sugar (helps with browning; optional but recommended)
- 2–4 tbsp water, as needed (to deglaze pan)
- 1 tsp balsamic vinegar (optional, for a final glossy, tangy finish)
For the Herb Mayo
- 1/2 cup (120 g) mayonnaise
- 2 tbsp finely chopped fresh flat-leaf parsley
- 1 tbsp finely chopped fresh chives (or finely sliced green onion tops)
- 1 tsp Dijon mustard
- 1 tsp freshly squeezed lemon juice
- 1 small garlic clove, finely minced (or 1/4 tsp garlic powder)
- 1/8 tsp fine sea salt, or to taste
- 1/8 tsp freshly ground black pepper, or to taste
For the Bagel Sandwiches
- 4 everything bagels, split horizontally
- 2 1/2 cups (about 12 oz / 340 g) cooked shredded roast chicken (rotisserie or leftover roasted chicken)
- 2 cups loosely packed baby spinach (about 2 oz / 60 g), washed and well dried
- 2 tsp olive oil (for brushing bagels or greasing the baking sheet)
- 4 slices provolone or Swiss cheese (about 4 oz / 115 g), optional but delicious
- Freshly ground black pepper, to finish
- Optional garnish: extra chopped parsley or chives; flaky sea salt
Step-by-Step Instructions
Step 1: Slice the onions and prep your ingredients
Peel the onions, then slice them in half from root to tip. Lay each half flat-side down and slice into thin, even crescent-shaped slices, about 1/8–1/4 inch thick. Even slices help the onions cook at the same rate without burning.
Chop the parsley and chives for the herb mayo, mince the garlic, and set them aside. Shred the cooked roast chicken into bite-sized pieces, discarding any skin or bones. Rinse and dry the baby spinach thoroughly (a salad spinner works well). Having everything prepped now makes assembling the sandwiches fast and relaxed later.
Step 2: Slowly caramelize the onions
In a large wide skillet, heat the butter and 1 tbsp olive oil over medium heat until the butter has melted and starts to foam. Add the sliced onions, sprinkle with 1/2 tsp salt and 1/2 tsp sugar, and toss to coat them in the fat.
Reduce the heat to medium-low. Cook the onions, stirring every few minutes, for 25–30 minutes. They will first soften and turn translucent, then gradually take on a deep golden-brown color and a jammy texture. If the pan starts to look dry or any brown bits threaten to burn, add 1–2 tbsp water at a time and gently scrape up the browned bits. Keep the heat on the low side; patience is what gives you sweet, rich flavor instead of burnt onions.
When the onions are medium to deep golden and very soft, stir in 1 tsp balsamic vinegar (if using). Taste and adjust salt. Turn off the heat and set aside.
Step 3: Make the herb mayo
While the onions cook, make the herb mayo. In a small bowl, combine the mayonnaise, chopped parsley, chopped chives, Dijon mustard, lemon juice, and minced garlic. Stir until smooth and evenly mixed.
Season with about 1/8 tsp salt and 1/8 tsp black pepper, then taste and adjust to your liking: more lemon for brightness, more herbs for freshness, or a pinch more salt if it tastes flat. Cover and refrigerate until you are ready to assemble the sandwiches. This allows the flavors to meld and intensify slightly.
Step 4: Prep the chicken and heat the oven
Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper or lightly grease it with a bit of olive oil.
Place the shredded roast chicken in a mixing bowl. If it seems a little dry (common with leftovers), you can drizzle on 1–2 tsp olive oil or a spoonful of chicken broth and toss lightly. Taste the chicken and add a small pinch of salt and pepper only if needed; rotisserie chicken is often already well-seasoned.
Set the chicken aside on the counter while the oven heats. Letting it lose some of its refrigerator chill will help it warm more evenly when the bagels go into the oven.
Step 5: Toast the bagels with the chicken (and cheese)
Arrange the split everything bagels on the prepared baking sheet, cut-side up. Lightly brush or drizzle the cut sides with the remaining 2 tsp olive oil.
Divide the shredded chicken evenly among the four bagel bottoms, piling it generously. If using cheese, lay 1 slice of provolone or Swiss over the chicken on each bottom half.
Place the baking sheet in the preheated oven and bake for 6–8 minutes, until the bagels are lightly crisped at the edges, the chicken is warmed through, and the cheese (if using) is fully melted and bubbly. The bagel tops should toast lightly as well on the same tray; if they brown too fast, you can flip them face-down halfway through.
Step 6: Assemble the caramelized onion & roast chicken bagels
Once the bagels and chicken are warm, remove the baking sheet from the oven. Spread a generous layer of herb mayo on the cut side of each bagel top. You can also smear a thin layer on the bagel bottoms around the chicken if you like extra sauce.
On each bagel bottom, layer a small handful of baby spinach over the warm chicken (and cheese). The heat will soften the leaves slightly without making them soggy. Spoon a generous mound of caramelized onions over the spinach, dividing them evenly between the four sandwiches.
Crown each with its bagel top, mayo side down. Gently press to help everything settle. If desired, slice each sandwich in half with a sharp serrated knife for easier handling. Finish with a crack of black pepper and a sprinkle of chopped herbs on top or on the serving plate for a fresh, vibrant look.
Step 7: Serve and enjoy
Serve the bagels immediately while they are warm and the onions are still glossy and soft. Add extra herb mayo and any remaining caramelized onions on the side for dipping or spooning over the top. These sandwiches are hearty enough to be a full meal on their own, but they also pair nicely with a simple green salad, crisp apple slices, or a cup of soup.
Pro Tips
- Go low and slow with the onions. If they are browning too fast or sticking hard to the pan, your heat is too high. Turn it down and add a splash of water to deglaze.
- Slice onions evenly. Uneven slices can lead to some bits burning before others are tender. Aim for consistent thickness so everything caramelizes at the same pace.
- Dry your spinach well. Excess water on the leaves can make the bottom of the bagel soggy. Spin or pat the spinach dry with a clean towel before assembling.
- Season thoughtfully. Rotisserie chicken, everything bagels, and cheese all bring salt. Taste each component before adding extra salt so the final sandwich stays balanced.
- Scale with ease. This recipe doubles well for a crowd. Caramelize a larger batch of onions in a wide Dutch oven, and bake multiple trays of bagels at once, rotating halfway through.
Variations
- Spicy kick: Stir 1–2 tsp sriracha, hot sauce, or finely chopped pickled jalapeños into the herb mayo for a gentle heat that cuts through the richness.
- Smoky bacon upgrade: Add 2 slices of crispy bacon per sandwich on top of the chicken before layering the spinach and onions for extra crunch and smoky flavor.
- Vegetarian twist: Swap the roast chicken for sautéed or roasted portobello mushrooms. Their meaty texture and umami flavor pair beautifully with the caramelized onions and herb mayo.
Storage & Make-Ahead
This sandwich is best enjoyed fresh, but several components can be prepared ahead:
The caramelized onions can be made up to 4–5 days in advance. Let them cool completely, then store in an airtight container in the refrigerator. Reheat gently in a skillet over low heat with a splash of water to loosen before using. The herb mayo can be made 2–3 days in advance and stored in a covered container in the fridge; give it a quick stir before spreading. Shredded roast chicken will keep for 3–4 days in the refrigerator in a sealed container.
Wait to toast the bagels and assemble the sandwiches until just before serving so they stay crisp and the spinach remains fresh. Fully assembled sandwiches do not store especially well, but if you have leftovers, wrap them tightly in foil and refrigerate for up to 1 day. Reheat in a 325°F (165°C) oven for about 10–12 minutes until warmed through; the spinach will wilt more, but the flavors will still be delicious.
Nutrition (per serving)
Approximate values per sandwich (1 loaded bagel, with cheese): about 680 calories; 34 g protein; 28 g fat; 64 g carbohydrates; 3 g fiber; 8 g sugar; 980 mg sodium. Actual values will vary based on the exact bagels, mayo, cheese, and chicken you use.
