Preheat your oven to 190°C (375°F). Rinse the peppers, slice off the tops, and carefully remove seeds and membranes to create a hollow cavity.
Heat a skillet over medium and add olive oil. Sauté chopped onions and minced garlic until fragrant and translucent, about 3 minutes, releasing a warm aroma and softening the onions.
Add the cooked quinoa, drained black beans, chopped vegetables, and dried oregano to the skillet. Stir well and cook for another 5-8 minutes until everything is heated through and slightly crispy around the edges, filling the air with a savory scent.
Season the mixture with salt and pepper to taste. Zest half the lemon directly over the filling, then squeeze in some lemon juice for brightness. Mix thoroughly to combine all flavors.
Use a spoon to generously stuff each hollowed-out pepper with the filling, pressing down slightly to pack it in. Place the stuffed peppers upright in a baking dish and drizzle a little olive oil over the tops.
Bake uncovered in the preheated oven for 25-30 minutes, until the peppers are tender and lightly browned around the edges, with bubbling filling that smells deeply savory.
Remove from the oven, let rest for 5 minutes to allow the filling to set, then garnish with chopped fresh herbs or an extra squeeze of lemon for added freshness.