01 -
Melt butter in a large skillet over medium heat; add sliced onions and cook, stirring frequently until golden brown and caramelized, about 25 minutes.
02 -
Season chicken breasts with salt and pepper. In a separate pan, sear chicken over medium-high heat until browned on both sides, about 3 minutes per side.
03 -
Add caramelized onions and thyme to the pan with chicken. Pour beef broth over the mixture, reduce heat to low, cover, and simmer for 15 minutes until chicken is cooked through.
04 -
Preheat the broiler. Place slices of Gruyère cheese atop each chicken breast and broil until cheese is melted and bubbly, about 2-3 minutes.