Preheat the oven to 375°F. Slice the tops off the bell peppers and remove the seeds and membranes. Use a small knife to carve simple jack-o'-lantern faces into the sides of each pepper.
In a large skillet over medium heat, cook the ground turkey with the diced onion and minced garlic until the turkey is no longer pink.
Stir the cooked rice, 3/4 cup of the tomato sauce, Italian seasoning, salt, and black pepper into the skillet. Mix until everything is evenly combined and warmed through.
Stand the carved peppers upright in a baking dish. Spoon the turkey and rice mixture into each pepper until filled.
Spoon the remaining tomato sauce over the top of each stuffed pepper, then sprinkle the shredded mozzarella cheese evenly across the openings.
Bake for 25 to 30 minutes until the peppers are tender and the cheese has melted.