Preheat oven to 375°F. Place hollowed peppers upright in a baking dish.
In a large skillet over medium heat, cook ground turkey, onion, and garlic until turkey is browned and onion is softened, about 7-8 minutes. Drain any excess fat.
Stir in cooked rice, diced tomatoes, Italian seasoning, salt, and pepper. Cook for 2-3 minutes until heated through.
Remove from heat and stir in ½ cup of the mozzarella cheese.
Spoon the filling mixture evenly into the four peppers, packing gently. Add ¼ cup water to the bottom of the baking dish.
Cover with foil and bake for 30 minutes. Remove foil, top each pepper with remaining cheese, and bake uncovered for 5 more minutes until cheese is melted.
Let cool for 5 minutes before serving. Garnish with fresh basil if desired.