Heat the oven: Preheat to 400°F (200°C).
Line a large baking sheet with parchment. Lightly oil a 9x13-inch baking dish.
Prep the peppers: Halve the bell peppers lengthwise and remove seeds and membranes. Brush or rub the cut sides with 1 tablespoon olive oil and a pinch of salt.
Arrange in the baking dish, cut side up.
Roast the sweet potato: Toss the diced sweet potato with 1 tablespoon olive oil, salt, and pepper. Spread on the lined baking sheet. Roast for 18–22 minutes, stirring once, until fork-tender and lightly caramelized.
Sauté aromatics: While the sweet potato roasts, warm 1 tablespoon olive oil in a large skillet over medium heat.
Add the red onion and a pinch of salt. Cook 4–5 minutes until softened. Stir in garlic and cook 30 seconds until fragrant.
Build the filling: Add cumin, chili powder, smoked paprika, oregano, and cinnamon to the skillet.
Toast the spices for 30 seconds. Stir in black beans, corn, drained tomatoes, and salsa. Simmer 3–4 minutes until slightly thickened.
Add the starch: Stir in the roasted sweet potato and cooked quinoa or brown rice (if using).
Taste and season with salt and pepper. The mixture should be well-seasoned and cohesive, not watery.
Stuff the peppers: Spoon the filling into each pepper half, packing it in. If using cheese, sprinkle some over the top.
Bake: Cover the dish loosely with foil and bake for 20 minutes.
Remove the foil and bake another 8–10 minutes, until the peppers are tender and the tops are lightly browned and bubbly.
Finish and serve: Garnish with cilantro or green onions. Serve hot with lime wedges and a dollop of Greek yogurt or sour cream if you like.