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Creamy French Onion Chicken Casserole

Creamy French Onion Chicken Casserole with caramelized onions, a thickened cream-and-broth sauce, and melty Gruyère topping. Chicken is browned briefly, baked until 165°F, then finished with French-fried onions for extra crunch.
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Prep Time 20 minutes
Cook Time 1 hour 10 minutes
Total Time 1 hour 30 minutes
Servings: 6 servings
Course: Dinner
Cuisine: French
Calories: 540
Ingredients Equipment Method Notes

Ingredients
  

Chicken
  • 1.5 lb boneless, skinless chicken breasts, cut into 1-inch pieces
  • 1 tbsp olive oil
  • 0.75 tsp kosher salt
  • 0.5 tsp black pepper
  • 0.5 tsp garlic powder
  • 0.5 tsp dried thyme
Caramelized onions
  • 2 tbsp unsalted butter
  • 3 large yellow onions, thinly sliced
  • 0.5 tsp kosher salt
  • 1 tsp brown sugar
  • 2 cloves garlic, minced
  • 1 tsp fresh thyme leaves, plus more for garnish
Creamy sauce
  • 2 tbsp all-purpose flour
  • 1.5 cup low-sodium chicken broth
  • 0.5 cup heavy cream
  • 1 tbsp Worcestershire sauce
  • 1 tsp Dijon mustard
  • 0.5 tsp black pepper
Topping
  • 1.5 cup shredded Gruyère cheese
  • 0.5 cup shredded mozzarella cheese
  • 1 cup French-fried onions
  • 1 tbsp chopped fresh parsley, optional

Equipment

  • 1 sheet pan
  • 1 cast iron skillet

Method
 

Prepare the oven and dish
  1. Preheat the oven to 375°F (190°C) and lightly grease a 9 × 13-inch baking dish. Make sure the dish is ready so you can transfer the chicken quickly after browning.
Season and brown the chicken
  1. Pat the chicken dry, then toss it with kosher salt, black pepper, garlic powder, and dried thyme. The pieces should look evenly coated.
  2. Heat olive oil in a large skillet over medium-high heat, then add the chicken and cook for 4–5 minutes, turning occasionally, until lightly golden. Transfer to the prepared baking dish without fully cooking through.
Caramelize the onions
  1. Reduce heat to medium-low and melt the butter in the same skillet. Let it melt and foam slightly before adding onions.
  2. Add the sliced onions, kosher salt, and brown sugar, then cook for 20–25 minutes, stirring every few minutes, until deeply golden, soft, and jammy. Keep the heat at medium-low to avoid scorching.
  3. Add the minced garlic and fresh thyme leaves, then cook for 30–60 seconds. The mixture should smell fragrant and look glossy.
Make the creamy French onion sauce
  1. Sprinkle the flour over the onions and stir for about 1 minute. The flour should lose its raw look.
  2. Slowly pour in the chicken broth while stirring to prevent lumps. The sauce should smooth out as it thickens.
  3. Add the heavy cream, Worcestershire sauce, Dijon mustard, and black pepper, then simmer for 3–5 minutes until thickened slightly. Stir occasionally so it stays creamy.
Assemble and bake
  1. Pour the creamy caramelized onion sauce evenly over the chicken. Covering the chicken helps everything cook through evenly.
  2. Sprinkle the Gruyère and mozzarella across the top. The cheese should form a visible layer.
  3. Bake uncovered for 20–25 minutes until the casserole is bubbling and the chicken reaches 165°F (74°C) internally. Look for steady bubbling around the edges.
  4. Sprinkle the French-fried onions over the cheese during the final 5 minutes of baking. They should turn lightly golden and stay crisp.
Rest and finish
  1. Let the casserole rest for 5–10 minutes. This helps the sauce set so it slices and serves cleanly.
  2. Finish with fresh thyme or parsley (optional). Add it right before serving for fresh color.

Notes

For best texture, don’t fully cook the chicken in the skillet—bake it until it reaches 165°F (74°C). Refrigerate leftovers in an airtight container for up to 4 days; reheat in the oven or microwave until hot. Freezing works for the casserole without the fresh-fried-onion topping—add the topping after reheating for crunch. Dietary swap: use gluten-free all-purpose flour for the roux to keep the sauce thick.