Place the quinoa and vegetable broth in a medium saucepan. Bring the mixture to a boil over medium-high heat. Lower heat to a simmer, cover the saucepan with a lid and cook covered until all the liquid is absorbed, 15 minutes. Allow the quinoa to rest for about 5 minutes, without opening the lid, then fluff with a fork.
Preheat oven to 375°F, and prepare the peppers by cutting them in half lengthwise and removing the seeds and membrane. Place the peppers in a baking dish cut side up, and pour water around the peppers — just enough to cover the bottom of the pan.
Heat olive oil in a large nonstick skillet over medium heat. Add onions and saute until they start to soften, about 2-3 minutes. Add the garlic and cook until fragrant, 1 more minute. Stir in the cooked quinoa, diced tomatoes, black beans and corn. Season with cumin, paprika, salt and pepper. Reduce heat to low and cook for additional 5 minutes, stirring frequently.
Carefully spoon the mixture into the sliced peppers and sprinkle the cheese on top.
Bake uncovered until the peppers are tender and the cheese is melted, about 30-35 minutes. Add optional toppings and serve hot.