# What You'll Need:
→ Chicken
01 - 4 large boneless, skinless chicken breasts
02 - 1 tablespoon olive oil
03 - 1 teaspoon salt
04 - ½ 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
09 - ¼ teaspoon sugar
10 - 1 teaspoon balsamic vinegar
11 - ½ cup low-sodium beef broth
→ Cheese
12 - 4 slices Gruyère cheese
13 - 4 tablespoons grated Parmesan cheese
→ Optional Garnish
14 - Fresh parsley, chopped
# Step-by-Step Guide:
01 - Set oven to 400°F (200°C).
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 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, about 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. Secure the opening with toothpicks if needed. Sprinkle grated Parmesan over the top.
06 - Heat olive oil in an oven-safe skillet over medium-high heat. Sear stuffed chicken breasts for 2–3 minutes per side until golden.
07 - Transfer skillet to the oven. Bake for 20–25 minutes, or until chicken is cooked through with an internal temperature of 165°F (74°C).
08 - Rest for 5 minutes before serving. Garnish with fresh parsley if desired.