Preheat your oven to 190°C (375°F). Using a sharp knife, carefully cut the tops off the bell peppers and remove all seeds and membranes, creating a clean cavity for stuffing.
Place the peppers upside down on a baking sheet and roast for about 10 minutes; this softens their skins and enhances flavor.
While the peppers are roasting, heat 2 tablespoons of olive oil in a pan over medium heat. Add the chopped onion and cook, stirring, until fragrant and slightly translucent, about 3 minutes.
Stir in the minced garlic and cook for another minute, until aromatic. Then add the cooked quinoa, black beans, corn, diced tomatoes, smoked paprika, oregano, salt, and pepper. Mix well and cook for 5 minutes, allowing flavors to meld and filling to heat through.
Remove the roasted peppers from the oven and increase oven temperature to 200°C (390°F). Carefully stuff each pepper with the quinoa mixture, pressing gently to pack the filling in.
Arrange the stuffed peppers upright in a baking dish, drizzle a little olive oil over the top, and bake for 25 to 30 minutes until the peppers are tender and slightly blistered on the surface.
Once baked, remove the peppers from the oven and let them sit for 5 minutes. Squeeze fresh lemon juice over the top for a bright, fresh flavor.
Garnish with freshly chopped herbs if desired, then serve warm and enjoy the layered, hearty flavors and vibrant colors.