Preheat the oven to 400°F (200°C) and lightly grease a 9×12-inch baking dish.

In a large skillet, melt the butter over medium-high heat. Add the sliced onions, salt, pepper, and half of the thyme. Cook until the onions soften, about 5-8 minutes.

Reduce the heat to medium and continue cooking, stirring occasionally, until the onions are beautifully caramelized (approximately 15 minutes). If the pan gets dry, add ¼ cup of beef broth.

Optional: Add balsamic vinegar to deglaze, scraping up any bits from the bottom, and let it reduce for about 1 minute.

Stir in the minced garlic, cooking just until fragrant, about 1 minute. Remove from heat and set aside.

Cut horizontal pockets into the chicken breasts without cutting all the way through. Season inside and out with salt, pepper, garlic powder, and ground thyme.

Generously stuff each chicken breast with the caramelized onions, Gruyère, and Parmesan cheese. Secure the opening with toothpicks.

Transfer any remaining onions to the prepared baking dish, mix with the remaining ¼ cup of beef broth, and place the stuffed chicken on top.

In the same skillet, heat olive oil over medium-high heat and sear the stuffed chicken for about 4 minutes on each side until golden brown.

Return the chicken to the baking dish, spooning over any pan juices. Sprinkle with the rest of the thyme and bake for 15-20 minutes, until the chicken reaches a safe internal temperature of 165°F (75°C).

Remove toothpicks before serving and enjoy warm alongside the delicious onions and sauce.