Preheat your oven to 400°F (200°C).
In a large skillet, heat the olive oil and butter over medium heat. Add the thinly sliced onions and a pinch of salt. Cook, stirring occasionally, until the onions are golden brown and caramelized, about 15-20 minutes.
While the onions are cooking, season the chicken breasts with salt, pepper, and garlic powder on both sides.
In your baking dish, place the seasoned chicken breasts in a single layer. Once the onions are caramelized, stir in the thyme and beef broth, then pour the onion mixture over the chicken breasts.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
After 25 minutes, remove the foil and sprinkle the shredded Gruyère cheese evenly over the chicken and onions. Return to the oven and bake for an additional 10 minutes, or until the chicken is cooked through and the cheese is melted and bubbly.
Once done, remove from the oven and allow to rest for a few minutes before serving. Garnish with fresh parsley if desired.