Heat olive oil or butter in a skillet over medium heat. Add sliced onions and cook, stirring occasionally, until they caramelize (about 15-20 minutes). Add minced garlic and balsamic vinegar or wine, if using, and cook for 1-2 more minutes. Set aside.

Season the Chicken:

Sprinkle chicken thighs or breasts with salt, black pepper, and dried thyme.

Layer the Crock Pot:

Place half of the caramelized onions on the bottom of the crock pot. Add the seasoned chicken on top, followed by the remaining onions. Pour in beef broth and add the bay leaf.

Cook:

Cook on low heat for 6-8 hours or high heat for 3-4 hours.

Ensure chicken reaches an internal temperature of 165°F (74°C).

Add Cheese and Broil:

Once the chicken is cooked, transfer it to an oven-safe dish. Spoon onions and broth over the chicken, then sprinkle shredded Gruyère cheese on top. Broil in the oven for 2-3 minutes until the cheese is bubbly and golden.

Serve:

Serve hot with mashed potatoes, rice, or crusty bread. Garnish with fresh thyme, if desired.