Prep the peppers. Heat your oven to 375°F (190°C).
Slice off the tops of the bell peppers and remove seeds and membranes. If a pepper won’t stand upright, trim a thin slice off the bottom to balance it.
Soften the peppers (optional but recommended). Place the peppers in a baking dish, drizzle with a little olive oil, and bake for 10 minutes. This jump-starts cooking so they’re tender by the end.
Sauté aromatics. In a large skillet, heat the olive oil over medium heat.
Add onion and cook 3–4 minutes until translucent. Stir in garlic and cook 30 seconds until fragrant.
Brown the turkey. Add the ground turkey, breaking it up with a spatula. Cook 5–7 minutes until no longer pink.
Season with salt, pepper, oregano, paprika, cumin, and red pepper flakes.
Add tomatoes and rice. Stir in the crushed tomatoes and tomato paste. Simmer 2–3 minutes to thicken slightly. Fold in the cooked rice until evenly combined.
Wilt the spinach. Add the chopped spinach to the skillet.
Cook 1–2 minutes until wilted (or warmed through if using frozen). Taste and adjust salt and spices.
Stuff the peppers. Spoon the turkey mixture into each pepper, packing it down gently. Fill to just below the rim so you have room for cheese later.
Bake covered. Add a splash of water (about 1/4 cup) to the baking dish to create steam.
Cover with foil and bake 20 minutes.
Add cheese and finish. Remove foil, top each pepper with shredded cheese, and bake uncovered for 8–10 more minutes until melted and lightly golden.
Garnish and serve. Let the peppers rest 5 minutes. Sprinkle with chopped parsley or basil and serve warm.