Preheat the oven to 400F.
Pound chicken breasts with a rolling pin until about 1” thick.
Season both sides of chicken with thyme, salt, and pepper.
Melt 1 tbsp of butter in an oven safe skillet then add the chicken.
Sear chicken for 3 minutes on each side. Set the seared chicken aside
In the skillet, heat 2 tbsp of butter over medium-high heat.
Once hot, add sliced onions and cook for 10-15 minutes or until browned and softened.
In a small bowl, mix the beef broth with 1 tbsp cornstarch.
Add the broth cornstarch mixture to the onions. Whisk together for 2-3 minutes or until thickened. Turn off heat.
Add chicken back to the skillet and using a spoon, cover the chicken with the onions and sauce.
Top each chicken breast with finely shredded gruyere cheese.
Transfer skillet to oven and bake for 5-8 minutes or until internal temp reads 165F.