Prep the Bell Peppers: Preheat your oven to 375°F (190°C). Bring a large pot of water to a boil. Cut the tops off the bell peppers, saving the tops, and remove seeds and membranes. Blanch the peppers in boiling water for 3-5 minutes to soften. Remove and drain upside down on a towel.

Cook Chicken Mixture: Heat the avocado oil in a large skillet over medium heat. Add the onions and garlic, cooking until soft and fragrant. Add the ground chicken, breaking it apart. Cook until browned and no longer pink. Season with salt and pepper.

Add Cauliflower Rice, Tomatoes, Marinara, and Seasonings: Stir in the cauliflower rice, diced tomatoes, marinara sauce, Italian seasoning, and crushed red pepper flakes into the skillet with the chicken mixture. Cook for 5-7 minutes, until the cauliflower rice is tender and the mixture has absorbed the flavors. Remove from heat and cool slightly before gently folding in the ricotta cheese.

Stuff the Peppers: Fill each blanched bell pepper with the chicken and cauliflower mixture. Place them on a casserole dish or cooking sheet.

Top with Cheeses: Sprinkle with Parmesan cheese, then cover with shredded mozzarella cheese.

Bake: Bake in the preheated oven for 20 minutes. Then, place the saved pepper tops on each pepper and continue baking for the last 10 minutes, or until the cheese is bubbly and golden, and the peppers are heated through.

Garnish and Serve: Let the peppers cool slightly, then garnish with fresh basil leaves. Serve warm.