# What You'll Need:
→ Chicken
01 - 4 large boneless, skinless chicken breasts
02 - 1 tablespoon olive oil
03 - 1 teaspoon salt
04 - 1/2 teaspoon black pepper
→ Onion Filling
05 - 2 large yellow onions, thinly sliced
06 - 2 tablespoons unsalted butter
07 - 2 cloves garlic, minced
08 - 1 teaspoon fresh thyme leaves or 1/2 teaspoon dried thyme
09 - 1/4 teaspoon sugar
10 - 1 teaspoon balsamic vinegar
11 - 1/2 cup low-sodium beef broth
→ Cheese
12 - 4 slices Gruyère cheese or Swiss cheese
13 - 4 tablespoons grated Parmesan cheese
→ Garnish
14 - Fresh parsley, chopped
# Method:
01 - Preheat oven to 400 degrees Fahrenheit.
02 - In a large skillet over medium heat, melt butter and add sliced onions. Cook, stirring often, for about 15 minutes until soft and golden brown. Add garlic, thyme, sugar, and a pinch of salt; cook 2 more minutes.
03 - Stir in balsamic vinegar and beef broth. Simmer until most of the liquid evaporates and onions are deeply caramelized, approximately 5 minutes. Remove from heat and let cool slightly.
04 - Pat chicken breasts dry. Using a sharp knife, carefully cut a pocket into the side of each breast, being careful not to cut all the way through.
05 - Season chicken breasts inside and out with salt and pepper. Fill each pocket with a generous spoonful of caramelized onions and a slice of Gruyère cheese.
06 - Secure the opening with toothpicks if needed. Sprinkle grated Parmesan over the top of each breast.
07 - Heat olive oil in an oven-safe skillet over medium-high heat. Sear stuffed chicken breasts for 2 to 3 minutes per side until golden brown.
08 - Transfer skillet to the oven. Bake for 20 to 25 minutes, or until chicken is cooked through with an internal temperature of 165 degrees Fahrenheit.
09 - Rest for 5 minutes before serving. Garnish with fresh parsley if desired.
