Heat the oven: Preheat to 375°F (190°C).
Lightly grease a baking dish large enough to hold all the peppers upright.
Cook the quinoa: Rinse quinoa under cold water. Combine with broth in a saucepan, bring to a boil, then reduce heat, cover, and simmer 15 minutes or until liquid is absorbed. Fluff with a fork and set aside.
Prep the peppers: Slice off the tops, remove seeds and membranes, and trim the bottoms if needed so they stand upright.
Place in the baking dish, open side up.
Sauté aromatics: Warm olive oil in a large skillet over medium heat. Add onion with a pinch of salt and cook 3–4 minutes until softened. Stir in garlic and cook 30 seconds until fragrant.
Brown the turkey: Add ground turkey, season with salt and pepper, and cook 5–6 minutes, breaking it up, until no pink remains.
Season and add tomatoes: Stir in cumin, smoked paprika, oregano, and red pepper flakes.
Add drained diced tomatoes and simmer 2–3 minutes.
Fold in quinoa and greens: Add cooked quinoa and chopped spinach. Stir until spinach wilts and everything looks evenly combined. Taste and adjust salt, pepper, or spices.
For brightness, add a squeeze of lemon or a splash of vinegar.
Fill the peppers: Spoon the filling into each pepper, packing gently and mounding slightly.
Bake covered: Tent the dish with foil and bake 25 minutes, until peppers are starting to soften.
Add cheese and finish: Remove foil, sprinkle cheese over the tops, and bake another 8–10 minutes until the cheese melts and the peppers are tender but not mushy.
Garnish and serve: Let cool 5 minutes. Sprinkle with fresh parsley or cilantro and serve warm.