Preheat the oven to 350 degrees F (175 degrees C).
Cut tops off green peppers and place peppers in a baking pan. Dice any pepper flesh surrounding stems; set aside. Discard seeds and stems.
Mix ground chicken, rice, onion, garlic, black pepper, and salt together in a bowl. Mix in diced peppers.
Scoop chicken mixture into the green peppers and cover with tomato sauce.
Bake in the preheated oven for 45 minutes. Top peppers with mozzarella cheese and continue to bake until filling is cooked through and cheese is golden, about 15 minutes longer.