Preheat and prep the pan. Heat your oven to 375°F (190°C). Lightly oil a baking dish that fits the peppers snugly so they stand upright.
Prepare the peppers. Slice off the tops about 1/2 inch down and remove seeds and membranes. If a pepper wobbles, shave a thin slice off the bottom so it stands without tipping.
Soften the peppers (optional but recommended). For more tender peppers, par-bake them empty for 10 minutes or microwave them in a covered dish with a splash of water for 3–4 minutes.
Drain any liquid.
Sauté the aromatics. Warm 1 tablespoon olive oil in a large skillet over medium heat. Add onion and cook 3–4 minutes until translucent. Stir in garlic and cook 30 seconds until fragrant.
Brown the chicken. Add ground chicken, breaking it up with a spoon.
Season with salt, pepper, Italian seasoning, smoked paprika, and red pepper flakes. Cook until no pink remains, about 5–6 minutes.
Build the filling. Stir in drained diced tomatoes and tomato sauce. Simmer 2–3 minutes.
Fold in the cooked rice (or quinoa). Taste and adjust seasoning. The mixture should be moist but not soupy.
Add cheese. Remove from heat and stir in half the shredded cheese.
This helps the filling bind and adds creaminess.
Stuff the peppers. Spoon the filling into each pepper, mounding slightly on top. Place them upright in the prepared baking dish. If you like, spoon a little extra tomato sauce over each for extra moisture.
Bake covered, then uncovered. Cover the dish with foil and bake 20 minutes.
Remove foil, sprinkle the remaining cheese over the tops, and bake another 10–15 minutes until the cheese melts and the peppers are tender.
Finish and serve. Let rest 5 minutes. Sprinkle with chopped parsley or basil. A light squeeze of lemon brightens everything.
Serve warm.