Step 1: Prepare the Onion Filling
• Caramelize the Onions: In a large skillet, heat the butter and olive oil over medium heat. Add the sliced onions and sugar (if using). Cook, stirring frequently, for about 15–20 minutes until the onions are soft and caramelized.
• Add Flavor: Stir in the garlic powder, thyme, and beef broth. Cook for an additional 5 minutes until most of the liquid evaporates. Season with salt and pepper.
• Mix in Cheese: Remove from heat and stir in the shredded Gruyère. Let cool slightly.

Step 2: Prepare the Chicken Breasts
• Preheat Oven: Preheat your oven to 375°F (190°C).
• Prepare Chicken: Place the chicken breasts on a cutting board and use a sharp knife to cut a pocket into each breast without cutting all the way through.
• Season: Season the chicken inside and out with salt and pepper.

Step 3: Stuff the Chicken
• Fill the Breasts: Spoon the onion-cheese mixture into each pocket.
• Optional Crunch: Roll the stuffed chicken in breadcrumbs if you’d like extra texture.

Step 4: Cook the Chicken
• Sear the Chicken: Heat the olive oil in an oven-safe skillet over medium-high. Add the stuffed chicken and sear for 3–4 minutes per side until golden.
• Bake: Transfer the skillet to the oven and bake 20–25 minutes, or until the chicken reaches 165°F (75°C). Add extra cheese on top during the last 5 minutes if desired.

Step 5: Serve
• Rest and Slice: Let the chicken rest for 5 minutes before cutting.
• Enjoy: Serve warm with pan juices.