Heat the oven to 375°F. Slice the tops off the bell peppers and remove the seeds and ribs. Arrange the peppers upright in a baking dish.
In a large bowl, combine the ground turkey, cooked brown rice, diced onion, egg, ½ cup of the tomato sauce, Worcestershire sauce, Italian seasoning, garlic powder, salt, and black pepper. Mix gently until evenly combined.
Divide the turkey mixture among the peppers, filling them without packing it too tightly. Pour the remaining ½ cup tomato sauce over the tops of the filling.
Cover the baking dish with foil and bake until the peppers are tender and the turkey is cooked through, about 45 to 55 minutes. The center of the filling should reach 165°F.
Remove the foil and sprinkle the mozzarella over the peppers. Bake uncovered for 5 to 8 minutes, until the cheese has melted. Let the peppers rest for a few minutes before serving.