Preheat oven to 375°F (190°C).
In a large non-stick skillet over medium-high heat, cook the ground turkey and diced onion until the turkey is fully browned and onions are translucent.
Stir in the cauliflower rice, tomato sauce, garlic powder, oregano, salt, and pepper; simmer for 5 minutes.
Arrange the bell peppers upright in a 9x9 inch baking dish.
Equally distribute the turkey and cauliflower mixture into the cavities of the bell peppers.
Add 0.25 cups of water to the bottom of the baking dish and cover tightly with aluminum foil.
Bake for 35 minutes until peppers are fork-tender.
Remove the foil, top each pepper with 2 tablespoons of shredded mozzarella cheese, and bake for an additional 10 minutes until the cheese is melted and bubbly.