Heat the oven: Preheat to 375°F (190°C).
Lightly oil a baking dish that can hold the peppers upright.
Prep the peppers: Slice off the tops, remove seeds and membranes, and trim the bottoms if needed so they stand. Brush the insides with a little olive oil and a pinch of salt.
Soften the peppers (optional but recommended): Place the peppers cut-side up in the baking dish, cover with foil, and bake for 10–12 minutes to soften. Remove and set aside.
Sauté aromatics: In a large skillet over medium heat, warm 2 tablespoons olive oil.
Add the diced onion and a pinch of salt. Cook 4–5 minutes until translucent. Stir in garlic and cook 30 seconds until fragrant.
Brown the turkey: Add ground turkey, breaking it up with a spoon.
Season with salt, pepper, oregano, smoked paprika, cumin, and red pepper flakes if using. Cook 5–7 minutes until no pink remains.
Add spinach: Stir in the spinach by handfuls. Cook until wilted (or fold in thawed, squeezed-dry frozen spinach).
Taste and adjust salt and pepper.
Mix in rice and sauce: Add the cooked rice and tomato sauce. Stir until combined and steamy. If it looks dry, add a splash more sauce or water.
You want a moist, spoonable filling.
Cheese it up: Stir in 1/2 cup of the shredded cheese to help bind the filling.
Stuff the peppers: Spoon the filling into each pepper, gently pressing down and mounding the tops.
Bake: Cover the dish with foil and bake 15 minutes. Remove foil, sprinkle the remaining cheese over the tops, and bake another 8–10 minutes until the cheese is melted and lightly golden.
Finish and serve: Let the peppers rest 5 minutes. Add a squeeze of lemon and a sprinkle of chopped parsley or basil.
Serve warm.