Step 1: Caramelize the Onions
Heat butter and olive oil in a medium sauté pan over medium heat.
Add the sliced onions, salt, and black pepper. Cook, stirring occasionally, until onions are golden and caramelized, about 20-25 minutes.
Step 2: Prepare the Chicken
Preheat your oven to 375°F (190°C).
Season chicken breasts with garlic powder, thyme, salt, and pepper.
In a large oven-safe skillet, sear the chicken on both sides over medium heat until golden brown, about 3-4 minutes per side. Remove from the skillet and set aside.
Step 3: Build the Dish
In the same skillet, add beef broth to deglaze the pan, scraping up any browned bits.
Add the caramelized onions back to the skillet, spreading them out evenly.
Place the seared chicken breasts on top of the onions.
Step 4: Add the Cheese
Sprinkle the Gruyère, mozzarella, and Parmesan cheese evenly over the chicken and onions.
Step 5: Bake the Dish
Transfer the skillet to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through (internal temperature of 165°F/74°C) and the cheese is melted and bubbly.
Step 6: Serve and Garnish
Remove from the oven and let rest for 5 minutes.
Sprinkle with fresh parsley before serving, if desired.