In a saucepan, bring the vegetable broth to a boil. Add the quinoa, reduce the heat to low, cover, and simmer for 15 minutes, or until the liquid is absorbed. Remove from heat and let it sit, covered, for 5 minutes. Fluff with a fork. Preheat your grill to medium-high heat for grilling, or your oven to 375°F for baking. Cut the tops off the bell peppers and remove the seeds and membranes. If necessary, slightly trim the bottoms to help them stand upright, but be careful not to create a hole.
In a skillet over medium heat, heat the olive oil. Add the onion and garlic, and sauté until soft. Add the zucchini and corn and cook for another 5 minutes. Stir in the black beans, cooked quinoa, cumin, paprika, salt, and pepper. Cook until everything is heated through. Remove from heat and stir in the cilantro. Stuff the bell peppers with the quinoa mixture and top with cheese if using.
If grilling, place the peppers on the grill, and close the lid. Grill for about 15 minutes, or until the peppers are tender and the cheese is melted. If baking, place the peppers in a baking dish, and bake for about 25 minutes. Serve the stuffed peppers with a side of your choice for a festive Fourth of July cookout.