Preheat your oven to 190°C (375°F). Line a baking sheet with parchment paper.
Slice off the tops of the bell peppers and carefully remove the seeds and membranes. Place the peppers cut side up on the prepared baking sheet.
In a skillet over medium heat, warm the olive oil. Add the diced onion and cook until translucent and fragrant, about 5 minutes, stirring occasionally.
Add the minced garlic to the skillet and cook for another 30 seconds until fragrant, stirring constantly.
Stir in the cooked quinoa, rinsed black beans, and toasted cumin. Cook for about 8-10 minutes, stirring occasionally, until everything is heated through and fragrant.
Season the filling with salt and pepper to taste, then remove from heat. Brighten the mixture with a splash of lemon juice and stir to combine.
Use a spoon to carefully stuff each pepper with the filling, pressing gently to pack it in.
Place the stuffed peppers on the baking sheet and bake for 25-30 minutes until the peppers are tender and blistered, with some charred spots on the skin.
Remove from the oven and let rest for about 5 minutes. Garnish with chopped cilantro or parsley for a fresh finish.
Serve the peppers hot, with an extra squeeze of lemon or a drizzle of vegan sour cream if desired, and enjoy their vibrant, flavorful goodness.