Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch (or similar) baking dish.
Step 2: Wash bell peppers, cut each in half lengthwise, and scoop out seeds and membranes. Arrange cut-side up in the baking dish.
Step 3: Heat olive oil in a large skillet over medium heat. Add chopped onion and cook until softened, about 5-7 minutes. Stir in minced garlic and cook for 1 minute until fragrant.
Step 4: Add diced zucchini, corn, and black beans to the skillet. Cook for 5-7 minutes, stirring occasionally, until vegetables are slightly tender. Stir in the cooked rice, crushed tomatoes, tomato paste, vegetable broth, oregano, cumin, smoked paprika, salt, and pepper. Mix well and adjust seasonings to taste.
Step 5: Generously spoon the rice and vegetable mixture into each bell pepper half. If using, sprinkle shredded cheese over the top of each stuffed pepper.
Step 6: Pour about 1/2 inch of water or vegetable broth into the bottom of the baking dish. Cover tightly with aluminum foil and bake for 30 minutes. Remove foil and bake for another 15-20 minutes, or until peppers are tender and the filling is bubbly and lightly browned.
Step 7: Let the vegetarian stuffed bell peppers with rice rest for 5-10 minutes before serving. Garnish with fresh parsley or cilantro, if desired.