Prep the peppers. Heat the oven to 375°F (190°C).
Slice the tops off the bell peppers and remove seeds and membranes. If they wobble, trim a very thin slice off the bottoms so they stand upright. Place them in a baking dish.
Soften the peppers (optional but helpful). Drizzle the peppers with a little olive oil and a pinch of salt.
Bake for 10 minutes while you prepare the filling. This jump-starts cooking so the peppers get tender without overbaking the filling.
Cook the aromatics. In a large skillet over medium heat, warm the olive oil. Add onion and a pinch of salt; cook 3–4 minutes until translucent.
Stir in the garlic and cook 30 seconds until fragrant.
Brown the meat. Add ground turkey or beef. Break it up with a spoon and cook until no longer pink, 5–7 minutes. Spoon off any excess grease if needed.
Season well. Sprinkle in the taco spice blend and stir to coat the meat.
Add the tomato paste and cook 1 minute to caramelize slightly.
Add mix-ins. Stir in diced tomatoes, black beans, and corn. If using rice or quinoa, add it now. Simmer 2–3 minutes to let flavors meld.
Taste and adjust salt and heat.
Make it creamy and high protein. Remove the pan from heat and fold in the Greek yogurt. This adds body and a slight tang while boosting protein. Stir in half the cheese.
Fill the peppers. Spoon the mixture evenly into the pre-baked peppers, mounding slightly.
Top each with the remaining cheese.
Bake. Return to the oven and bake 15–20 minutes, until the peppers are tender and the cheese is bubbly and golden in spots.
Finish and serve. Let the peppers rest 5 minutes. Top with cilantro, green onions, avocado, and a squeeze of lime. Serve with salsa and extra yogurt if you like.