French Onion Stuffed Chicken Casserole

French Onion Stuffed Chicken Casserole makes for a delicious dinner! Juicy, succulent chicken breasts stuffed with caramelized onions and glorious melted cheese. A perfect weeknight or weekend dinner. Low Carb and Keto approved!

2 tablespoons unsalted butter
4 large onions (halved and thinly sliced)
Pinch of salt and pepper (to season)
2 teaspoons fresh chopped thyme (divided)
2 tablespoons white wine, sherry or balsamic vinegar ((OPTIONAL))
2 cloves garlic (minced)
1/2 cup beef broth or stock (divided)
1 tablespoon olive oil
4 boneless, skinless chicken breasts
2 teaspoons garlic powder
1 teaspoon ground thyme
1 cup shredded gruyere cheese ((or mozzarella cheese))
4 tablespoons freshly grated parmesan cheese