Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Add the bell peppers and cook for 3 minutes to soften slightly. Remove and drain upside down on paper towels.
Heat olive oil in a large skillet over medium heat. Add the chopped onion and cook for 3 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Add ground turkey to the skillet. Cook, breaking it up with a spoon, until browned and cooked through, about 6-8 minutes.
Stir in cauliflower rice, diced tomatoes, Italian seasoning, salt, and black pepper. Cook for 3 minutes until heated through and well combined.
Remove skillet from heat and stir in half of the mozzarella cheese and all of the Parmesan cheese.
Place the bell peppers upright in a baking dish. Spoon the turkey mixture evenly into each pepper.
Sprinkle the remaining mozzarella cheese on top of the stuffed peppers.
Cover the baking dish with foil and bake for 25 minutes.
Remove the foil and bake for an additional 10 minutes until the cheese is melted and bubbly.
Garnish with chopped fresh parsley before serving.