Name: Cheesy French Onion Chicken Sandwich
Prep time: 20 min
Cook time: 45 min
Total time: 65 min
Ingredients: 3 large yellow onions (thinly sliced); 1 tbsp butter; 1 tbsp olive oil; 1 tsp salt; 1 tsp sugar; 2 sprigs fresh thyme; 1/4 cup beef broth; 1 tbsp balsamic vinegar; 2 boneless skinless chicken breasts; 1 tsp garlic powder; 1 tsp onion powder; 1 tsp smoked paprika; 1 tsp salt; 1 tsp pepper; 1 tbsp olive oil; 1/2 cup mayo; 2 cloves garlic (minced); 1 tsp dijon mustard; 1 tsp lemon juice; 1 finely chopped fresh thyme; 1 to taste salt & pepper; 1 cup beef broth; 1 tsp Worcestershire; 2 cloves garlic (minced); 2 sprigs thyme; 1 pinch salt & pepper; 4 thick slices sourdough or ciabatta; 8 oz gruyère cheese (shredded); 2 slices Swiss cheese

This cheesy French onion chicken sandwich is French onion soup in sandwich form, and it is unreal. Seasoned chicken breasts are seared and topped with melty Swiss, then piled onto Gruyère-toasted sourdough with garlic thyme aioli and a mountain of jammy caramelized onions. A quick au jus for dipping makes this French onion chicken sandwich next-level cozy.

It is a weekend project that is completely worth it. Love caramelized onions? Try my Caramelized Onion Grilled Cheese next.

Why You’ll Love This French Onion Chicken Sandwich

Jammy caramelized onions with balsamic and thyme.
Gruyère and Swiss for a double cheese melt.
Garlic thyme aioli on toasted bread.
Au jus for dipping, just like French dip.

Ingredients for a French Onion Chicken Sandwich

The Caramelized Onions

Thinly sliced yellow onions, butter, olive oil, salt, sugar, fresh thyme, beef broth and balsamic vinegar.

The Chicken

Boneless, skinless chicken breasts seasoned with garlic powder, onion powder, smoked paprika, salt and pepper, seared in olive oil and topped with Swiss cheese.

The Garlic Aioli

Mayo, garlic, Dijon, lemon juice, fresh thyme, salt and pepper.

The Au Jus and Bread

Beef broth, Worcestershire, garlic and thyme, plus thick slices of sourdough or ciabatta toasted with Gruyère.

Swap note: Use gluten-free bread and check your Worcestershire and broth labels for gluten free. Chicken thighs work great in place of breasts.

How to Make a French Onion Chicken Sandwich

Step 1: Make the aioli

Whisk together the aioli ingredients and chill while you cook.

Step 2: Caramelize the onions

Melt the butter with olive oil over medium-low heat. Add the onions, salt, sugar and thyme and cook 30 to 35 minutes, stirring occasionally, until deeply golden and jammy. Deglaze with the beef broth and balsamic and cook until mostly reduced.

Step 3: Sear the chicken

Season the chicken with garlic powder, onion powder, smoked paprika, salt and pepper. Sear in butter and olive oil over medium-high heat 3 to 4 minutes per side until cooked through. Top with Swiss in the last minute, cover to melt and let it rest.

Step 4: Make the au jus

In the same pan, add the beef broth, Worcestershire, garlic and thyme. Simmer 5 minutes, scraping up the fond, then pour into a bowl for dipping.

Step 5: Toast the bread

Brush the bread with oil or butter, top with Gruyère and toast at 350°F for 3 to 5 minutes.

Step 6: Build and serve

Spread aioli on the toasted bread, add the chicken and pile on the caramelized onions. Serve with hot au jus for dipping.

Carly’s Expert Tips

Slice the onions evenly so they caramelize at the same rate.
Don’t rush the onions. Low heat makes them sweet and jammy.
Let the chicken rest so it stays juicy when sliced.
Toast the bread last so it stays crispy.

Make It Your Own

Chicken thighs: Use boneless thighs for extra juicy chicken.
Roast beef: Swap the chicken for thinly sliced roast beef.
Provolone: Use provolone instead of Swiss.
Mushrooms: Add sautéed mushrooms with the onions.

Make-Ahead and Meal Prep: Caramelize the onions and make the aioli up to 3 days ahead. Then the sandwiches come together in about 20 minutes.

What to Serve With a French Onion Chicken Sandwich

Serve with the au jus for dipping, plus fries, a crisp green salad or Roasted Tomato Basil Soup.

Storing and Reheating

Store leftover chicken, onions and au jus separately in the fridge for up to 3 days.

Reheat the chicken and onions in a skillet and toast fresh bread before assembling.

What People Ask About French Onion Chicken Sandwich

How long does it take to caramelize onions?

About 30 to 35 minutes over medium-low heat.

What cheese goes on a French onion chicken sandwich?

Gruyère and Swiss give that classic French onion soup flavor.

Can I make the caramelized onions ahead?

Yes. They keep up to 5 days in the fridge.

What is au jus?

A light, savory broth made from pan drippings for dipping.