Preheat your oven to 350°F (175°C).
Slice the bell peppers in half lengthwise and remove the seeds. Set aside.
In a large skillet, heat a splash of olive oil over medium heat. Add the chopped onion and minced garlic and sauté until the onion is translucent (about 3-5 minutes).
Add the ground beef (or turkey) to the skillet, breaking it apart with a spoon. Cook until browned and fully cooked, about 7-10 minutes. Drain any excess fat.
Stir in the black beans, corn, diced tomatoes, cumin, chili powder, salt, and pepper. If using rice, mix it in at this step. Cook for another 5 minutes.
Carefully spoon the mixture into the prepared bell pepper halves and place them upright in the baking dish.
Cover the dish with aluminum foil and bake for 30 minutes.
Remove the foil, sprinkle cheese on top of the peppers, and bake for an additional 15 minutes, until the cheese is melted and bubbly.
Remove from the oven and allow to cool for a few minutes before serving.