Prep the peppers: Slice about 1/2 inch off the tops, remove seeds and membranes, and trim the bottoms if needed so they stand upright. Lightly brush the outsides with olive oil.
Preheat the air fryer: Set it to 360°F (182°C) for 3–5 minutes so the basket is hot.
Sauté aromatics: Heat the olive oil in a skillet over medium. Cook onion for 3–4 minutes until soft, then add garlic for 30 seconds until fragrant.
Brown the meat: Add ground beef or turkey to the skillet. Season with salt, pepper, Italian seasoning, paprika, and red pepper flakes if using. Cook, breaking it up, until no longer pink.
Add veggies and sauce: Stir in marinara, riced cauliflower, and spinach. Cook 2–3 minutes to meld flavors. Taste and adjust salt and pepper.
Add cheese: Remove from heat. Stir in 1/2 cup mozzarella and the Parmesan until combined.
Stuff the peppers: Spoon the filling into each pepper, packing it down slightly. Top with the remaining 1/4 cup mozzarella.
Air fry: Arrange peppers upright in the basket (use a small piece of foil ring under a pepper if it wobbles). Cook at 360°F (182°C) for 10–14 minutes, until peppers are tender and cheese is melted and browned in spots.
Finish and serve: Let rest 3–5 minutes. Garnish with chopped parsley or basil. Serve hot.
