Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat.
Add chopped onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Add ground turkey to the skillet and cook until browned and cooked through, breaking it apart with a spatula, about 6-8 minutes.
Stir in cauliflower rice, diced tomatoes, dried oregano, dried basil, smoked paprika, salt, black pepper, and crushed red pepper flakes (if using). Cook for another 5 minutes, stirring occasionally.
Remove skillet from heat and stir in half of the mozzarella cheese and half of the Parmesan cheese until combined.
Place the hollowed bell peppers upright in a baking dish.
Spoon the turkey mixture evenly into each bell pepper, packing gently.
Top each stuffed pepper with the remaining mozzarella and Parmesan cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 5-7 minutes until the cheese is melted and slightly golden.
Remove from oven and garnish with chopped fresh parsley before serving.