Preheat oven to 375°F (190°C).
Heat olive oil and butter in a large skillet over medium heat.
Add the sliced onions and a pinch of salt, cooking until caramelized.
Season the chicken breasts with salt and pepper.
Push the onions to the side and add the chicken to the skillet.
Sear the chicken for 5-7 minutes on each side until golden.
Pour in the beef broth and sprinkle thyme over the top.
Transfer the skillet to the preheated oven and bake for 20-25 minutes.
Remove from the oven and top the chicken with caramelized onions and Gruyère cheese.
Return to the oven for an additional 5-10 minutes until the cheese is melted and bubbly.
Serve hot, garnished with fresh thyme if desired.