Preheat oven to 400°F; line a baking sheet with parchment.

With a sharp knife, carefully cut each breast horizontally, leaving one edge intact; open like a book.

Season inside and outside with garlic powder, salt, and pepper.

Fill with mozzarella, tomatoes, and basil; fold closed and secure with toothpicks if needed.

Heat oil in an oven-safe skillet over medium-high; sear chicken 2-3 minutes per side until golden.

Transfer skillet to oven; bake 12-15 minutes until chicken reaches 165°F internally.

Remove from oven; drizzle with balsamic glaze and rest 5 minutes before serving.