Heat the oven: Preheat to 375°F (190°C).
Lightly oil a 9x13-inch baking dish.
Cook the quinoa: Rinse the quinoa under cold water. Cook according to package directions with a pinch of salt. Fluff and set aside.
You’ll need about 3 cups cooked quinoa.
Prep the peppers: Slice the tops off the bell peppers and scoop out seeds and membranes. If they wobble, shave a thin slice from the bottoms so they stand upright. Place them in the baking dish cut-side up.
Sauté the aromatics: In a large skillet, heat olive oil over medium.
Add onion and a pinch of salt. Cook until translucent, 3–4 minutes. Stir in garlic and cook 30 seconds until fragrant.
Brown the turkey: Add ground turkey to the skillet.
Break it up with a spoon and cook until no longer pink, 5–7 minutes. Season with cumin, smoked paprika, oregano, chili powder (if using), salt, and pepper.
Add veggies and tomatoes: Stir in zucchini, diced tomatoes with their juices, and tomato paste. Simmer 3–4 minutes to marry flavors and thicken slightly.
Combine with quinoa: Remove from heat.
Fold in the cooked quinoa and half the chopped herbs. Squeeze in a little lemon juice for brightness. Taste and adjust seasoning; the filling should be well-seasoned.
Stuff the peppers: Spoon the filling into each pepper, mounding slightly.
Sprinkle the tops with about half the cheese.
Add moisture to the pan: Pour the chicken broth or water into the bottom of the baking dish (around the peppers, not over them). This helps the peppers steam and become tender.
Bake covered: Cover the dish tightly with foil and bake for 25–30 minutes, until the peppers are starting to soften.
Finish with cheese: Remove foil, sprinkle on the remaining cheese, and bake uncovered for 8–10 minutes more, until the cheese melts and the peppers are tender but not collapsing.
Garnish and serve: Let rest 5 minutes. Sprinkle with remaining herbs.
Serve warm with a fresh side salad or a dollop of plain Greek yogurt.