Preheat your oven to 400°F (200°C) and grease a 9x12-inch baking dish.

Melt 2 tablespoons of unsalted butter in a skillet over medium-high heat and sauté sliced onions with salt, pepper, and half the thyme for 5–8 minutes.

Reduce heat to medium and cook onions for about 15 minutes, stirring occasionally until they are caramelized.

Optional: Deglaze the pan with a splash of white wine or balsamic vinegar and stir for about a minute.

Stir in minced garlic and cook for about 1 minute, then remove from heat to cool.

Slice chicken breasts horizontally to create pockets, season inside and outside with salt, pepper, garlic powder, and ground thyme.

Stuff each chicken pocket with the onion mixture and shredded Gruyere cheese, securing with toothpicks.

Transfer remaining caramelized onions to the baking dish and mix in 1/4 cup of beef broth.

Sear the stuffed chicken in a skillet with olive oil for 4 minutes on each side until golden brown.

Place the seared chicken in the baking dish, top with leftover onions and thyme, then bake for 15–20 minutes until chicken reaches 165°F (74°C).

Once baked, discard toothpicks and serve hot with pan juices and onions.