Preheat your oven to 375°F (190°C). Place the hollowed-out bell peppers in a baking dish, cut-side up, and set aside.
In a medium saucepan, bring 2 cups of vegetable broth or water to a boil. Add the rinsed quinoa, reduce heat to low, cover, and simmer for 12-15 minutes until the quinoa is tender and the liquid is absorbed. Fluff with a fork and set aside.
Heat the olive oil in a large skillet over medium heat. Add the chopped onion and sauté for 2-3 minutes until softened. Stir in the minced garlic and cook for 1 minute until fragrant.
Add the diced zucchini to the skillet and cook for 3-4 minutes until tender. Stir in the black beans, diced tomatoes, cooked quinoa, cumin, smoked paprika, salt, and black pepper. Cook for another 2-3 minutes, stirring to combine the flavors.
Spoon the quinoa mixture into the bell peppers, pressing down gently to fill them evenly. If using cheese, sprinkle it over the tops of the stuffed peppers. Cover the baking dish with foil and bake for 30 minutes. Remove the foil and bake for an additional 5-10 minutes, until the peppers are tender and the cheese (if used) is melted and bubbly.
Remove from the oven and let the peppers cool slightly. Garnish with fresh cilantro or parsley if desired. Serve your quinoa stuffed bell peppers warm and enjoy a colorful, hearty dinner!