01 -
Preheat oven to 400°F (200°C) and lightly grease a 9x12-inch baking dish. Set aside.
02 -
Melt butter in a large skillet over medium-high heat. Add onions, season with salt, pepper, and half of the fresh thyme, and cook for 5-8 minutes until soft. Reduce heat to medium and cook, stirring occasionally, until onions become caramelized and jammy (about 15 minutes). Pour in 1/4 cup of broth if the pan becomes too dry, and stir until browned.
03 -
Pour white wine, sherry, or balsamic vinegar into the pan to deglaze and enhance flavors. Let cook for 1 minute until the sauce thickens slightly.
04 -
Stir in minced garlic and cook for about 1 minute until fragrant. Turn off heat and let cool slightly.
05 -
Cut chicken breasts horizontally to create pockets. Season chicken all over and inside the pockets with salt, pepper, garlic powder, and ground thyme.
06 -
Spoon 1-2 tablespoons of caramelized onions into each pocket and stuff each chicken breast with gruyere and parmesan cheeses. Secure the openings of the breasts with toothpicks.
07 -
Transfer the remaining caramelized onions from the skillet to the prepared baking dish. Mix in the remaining 1/4 cup of beef broth. Set aside.
08 -
Heat olive oil in the same skillet over medium-high heat. Sear the stuffed chicken breasts for about 4 minutes each side until golden brown. The chicken will not be fully cooked yet.
09 -
Place the seared chicken on top of the caramelized onions in the baking dish. Spoon some sauce over each chicken breast, top with the remaining fresh thyme, and bake for 15-20 minutes until fully cooked.
10 -
Discard toothpicks and serve the chicken with pan juices and caramelized onions.