Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x9 inch baking dish.
Step 2: Slice the tops off the bell peppers and remove the seeds and membranes. If the peppers won't stand up, slice a tiny bit off the bottom to level them without creating a hole.
Step 3: In a large skillet, heat olive oil over medium heat. Sauté the onion for 5 minutes until soft, then add the garlic and cook for another minute.
Step 4: Stir in the cooked rice, black beans, corn, salsa, cumin, and chili powder. Cook for 2-3 minutes until heated through. Season with salt and pepper.
Step 5: Stir half of the shredded cheese into the rice mixture until partially melted.
Step 6: Spoon the filling into the prepared bell peppers, packing it down firmly. Place the peppers upright in the baking dish.
Step 7: Pour 1/2 cup of water into the bottom of the dish. Cover with aluminum foil and bake for 35 minutes.
Step 8: Remove the foil, sprinkle the remaining cheese over the tops of the peppers, and bake for an additional 10 minutes until the cheese is bubbly and the peppers are tender.