Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish or a similar-sized casserole dish.2. Cut the 4 green peppers in half lengthwise. Remove all seeds and white membranes from inside the peppers. Place the pepper halves into the greased baking dish with the cut side facing up. Bake the peppers for 15 to 20 minutes. They should become slightly soft.3. While the peppers bake, prepare the filling. Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the 1 small finely diced onion to the skillet. Cook the onion for 3 to 4 minutes until it softens. Add the 2 cloves of minced garlic. Cook for 1 more minute until it smells fragrant.4. Stir in 1 cup of quinoa, 1 can of black beans, 1 cup of sweetcorn, 1 can of diced tomatoes, 1 teaspoon of cumin, chili powder, smoked paprika, salt, and pepper into the skillet. Mix all ingredients well. Cook this mixture for 5 to 7 minutes until it is heated through. Remove the skillet from the heat. If you are using cheese, stir in 1 cup of the shredded Mexican cheese into the filling mixture now.5. Carefully remove the partially baked bell peppers from the oven. Evenly fill each bell pepper half with the quinoa and black bean mixture. If you are using more cheese, sprinkle the remaining 0.5 cup of shredded Mexican cheese over the top of each stuffed pepper.6. Return the stuffed peppers to the oven. Bake for an additional 15 to 20 minutes. The cheese should be melted and bubbly. The peppers should be tender.7. Remove the stuffed peppers from the oven. Let them cool for 5 minutes before serving. Garnish with fresh chopped cilantro, if desired.