Preheat your oven to 375°F (190°C).
In a skillet, heat olive oil over medium heat and add the sliced onions.
Sprinkle the onions with garlic powder, salt, and pepper, cooking for about 10-15 minutes.
While the onions are caramelizing, prepare the chicken breasts by placing them between two sheets of plastic wrap and gently pounding them to an even thickness.
Once the onions are caramelized, remove them from heat and mix in the mozzarella cheese.
Lay each chicken breast flat and spoon the onion and cheese mixture into the center.
Roll the chicken tightly around the filling and secure with toothpicks or kitchen twine.
Place the stuffed chicken rolls seam-side down in a baking dish.
Bake in the preheated oven for 25-30 minutes or until chicken is cooked through.
Remove from the oven, let rest for a few minutes, and then remove toothpicks before serving.
