Step 1: Prepare the Peppers
Preheat your oven to 375°F (190°C). Cut the tops off the bell peppers and remove the seeds and white membranes. If needed, slice a very thin piece off the bottom of each pepper to help them stand upright, being careful not to cut through the cavity. Brush the outside of the peppers lightly with olive oil and season the insides with a pinch of salt and pepper. Place them upright in a baking dish just large enough to hold them snugly.

Step 2: Par-Cook the Peppers (Optional but Recommended)
Pour about ¼ inch of water into the bottom of the baking dish. Cover the dish tightly with aluminum foil. Bake for 15-20 minutes. This step softens the peppers slightly, ensuring they become perfectly tender and fully cooked by the time the filling is done. Remove from the oven and carefully drain any water from the dish. Keep the oven on.

Step 3: Cook the Filling
While the peppers par-cook, heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the diced onion and cook for 4-5 minutes until softened. Add the minced garlic, oregano, basil, fennel seeds (if using), and red pepper flakes. Cook for 1 more minute until fragrant. Add the ground chicken, breaking it up with a spoon, and cook until no longer pink, about 6-8 minutes. If using pre-cooked shredded chicken, simply add it to the skillet to warm through.

Step 4: Combine the Filling Ingredients
To the skillet, add the marinara sauce, cooked rice (or alternative), ¼ cup of the Parmesan cheese, and chopped parsley. Stir until everything is well combined and heated through. Season the mixture generously with salt and black pepper to taste. Remove from heat.

Step 5: Stuff the Peppers
Spoon the warm chicken and rice filling evenly into the par-cooked peppers, packing it down gently. Fill them to the top.

Step 6: Add the Cheese Topping
In a small bowl, mix the shredded mozzarella with the remaining ¼ cup of Parmesan. Sprinkle the cheese mixture generously over the top of each stuffed pepper. For a crispy topping, you can sprinkle the optional buttered breadcrumbs over the cheese.

Step 7: Bake Until Bubbly and Golden
Return the baking dish to the oven (uncovered) and bake for 20-25 minutes, or until the cheese is melted, bubbly, and starting to brown, and the peppers are tender when pierced with a fork.

Step 8: Rest, Garnish, and Serve
Remove from the oven and let the peppers cool in the dish for 5-10 minutes. This allows the filling to set for easier serving. Garnish with fresh basil or parsley. Serve hot, one pepper per person.