Preheat your oven to 375°F (190°C).
In a large skillet, heat the olive oil over medium heat. Add the sliced onions and a pinch of salt. Sauté the onions, stirring occasionally, until they are caramelized, about 10-15 minutes.
While the onions are cooking, season the chicken breasts with salt, pepper, garlic powder, and dried thyme on both sides.
Remove the caramelized onions from the skillet and set aside. In the same skillet, sear the seasoned chicken breasts until browned, about 3-4 minutes on each side.
In the baking dish, combine the seared chicken, caramelized onions, beef broth, and Worcestershire sauce. Ensure the chicken is well-coated with the mixture.
Cover the dish with aluminum foil and bake in the preheated oven for 20 minutes.
After 20 minutes, remove the foil and sprinkle the shredded Swiss cheese over the chicken. Return to the oven and bake uncovered for an additional 10 minutes, or until the cheese is melted and bubbly.
Once done, remove from the oven and let it rest for a few minutes. Garnish with chopped parsley before serving.