Preheat the oven to 375°F (190°C).
In a skillet, heat the olive oil over medium heat. Add the sliced onions and sauté until they are caramelized and translucent, about 8 to 10 minutes.
While the onions are cooking, season the chicken breasts with garlic powder, thyme, salt, and pepper on both sides.
In the bottom of the baking dish, pour half of the French onion soup. Place the seasoned chicken breasts on top of the soup.
Once the onions are caramelized, layer them evenly over the chicken breasts.
Pour the remaining French onion soup over the onions and chicken, ensuring everything is covered.
Sprinkle the shredded Gruyère cheese on top of the chicken and soup mixture.
Cover the baking dish with aluminum foil and place it in the preheated oven. Bake for 20 minutes.
After 20 minutes, remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.
Remove the dish from the oven and let it rest for a few minutes before serving. Garnish with fresh parsley if desired.