Preheat your oven to 375°F.
Place one chicken breast on a cutting board and hold it flat with your hand.
Use a sharp knife to cut a pocket into the side, but do not cut all the way through.
Slide one slice of mozzarella into the pocket.
Repeat with the remaining chicken breasts.
Rub both sides with olive oil.
Sprinkle salt, pepper, garlic powder, and Italian seasoning evenly over each piece.
Set the chicken in a baking dish.
Bake for 25–30 minutes, until the chicken reaches 165°F inside.
Spoon marinara on top before serving if you like.