Preheat oven to 350°F. Bring a large pot of water to boil.
Cut the peppers in half vertically right through the middle of the stem and the bottom of the pepper. Remove the membranes and seeds.
Parboil the peppers in the pot of water for 3–4 minutes until slightly softened. Drain and place the peppers cut side up in a large baking dish.
In a large skillet, heat the oil over medium heat. Add the onion and sauté for 4 minutes or until translucent. Add the garlic and cook for 1 minute further.
Add the ground beef and cook, stirring constantly to break up the meat, for 5 minutes or until browned.
Stir in the chili powder, cumin, green chiles, cooked rice, tomatoes, black beans and corn. Season generously with salt and pepper.
Spoon the ground beef rice mixture into the peppers and sprinkle the tops with cheese.
Bake for 15-20 minutes until peppers are tender and cheese is bubbling. Serve warm, garnished with green onions or cilantro if using.