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