Preheat oven to 375°F (190°C). Pat chicken dry and slice a pocket in each breast.
Season inside and out with salt, pepper, garlic powder, and Italian seasoning.
Spoon about 2 tbsp pesto into each pocket, then add mozzarella. Close with toothpicks if needed.
Lightly spray or oil a baking dish and arrange chicken in a single layer.
Bake 22–28 minutes, until chicken reaches 165°F (74°C) and cheese is melted; broil 1–2 minutes if you want a browned top.
Let rest 5 minutes before serving and remove toothpicks.