Preheat the oven to 375°F (190°C). In a large skillet, melt the butter with olive oil over medium heat. Add the thinly sliced onions and cook slowly, stirring occasionally, for about 20–25 minutes. The onions should turn soft, golden brown, and deeply caramelized. Stir often to prevent burning and adjust the heat if needed — this slow cooking develops the signature sweetness and depth of flavor.
Once the onions are caramelized, stir in the thyme and pour in the beef broth. Let the mixture simmer for 5 minutes to blend the flavors. For a thicker sauce, whisk in the flour and cook for an additional 2–3 minutes until slightly thickened. Remove from the heat and set aside to cool slightly while you prepare the chicken.
2️⃣ Prepare and Stuff the Chicken
Using a sharp knife, carefully cut a pocket into the side of each chicken breast, keeping the knife parallel to the cutting board. Be cautious not to slice all the way through. This pocket will hold the caramelized onion and cheese filling.
Season the chicken on both sides and inside the pocket with garlic powder, onion powder, salt, and black pepper.
Fill each chicken breast with a few spoonfuls of the caramelized onion mixture and a generous portion of shredded Gruyère cheese. Secure the openings with toothpicks if necessary to keep the filling from spilling out during cooking.
