Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). Lightly grease a baking dish.
Season the chicken breasts on both sides with salt and pepper.
Heat the olive oil in a large oven-safe skillet over medium heat. Add the sliced onions and cook, stirring occasionally, for 10 to 15 minutes until they are soft and beginning to caramelize.
Add the minced garlic and dried thyme to the onions and cook for 1 minute until fragrant.
Pour in the beef broth and Worcestershire sauce. Bring the liquid to a simmer and cook for 2 minutes, scraping up any browned bits from the bottom of the skillet.
Nestle the seasoned chicken breasts into the onion and broth mixture in the skillet.
Transfer the skillet to the preheated oven. Bake for 15 minutes.
Remove the skillet from the oven. Sprinkle the shredded Gruyère cheese evenly over the top of the chicken.
Return the dish to the oven and bake for another 5 to 8 minutes, or until the chicken is cooked through (internal temperature reaches 165 degrees Fahrenheit) and the cheese is melted and bubbly.
Remove from the oven, let it rest for 5 minutes, and garnish with fresh parsley before serving.
Notes
If you do not have an oven-safe skillet, transfer the chicken and onion mixture to a standard baking dish after simmering on the stovetop.
For deeper onion flavor, caramelize the onions slowly over low heat for up to 25 minutes.
You can substitute Provolone or Swiss cheese if Gruyère is unavailable.