Preheat your oven to 190°C (375°F). Slice off the tops of the peppers and carefully scoop out the seeds and membranes with a spoon, creating hollow cavities.
Heat olive oil in a large skillet over medium heat. Add the chopped onion and minced garlic, sautéing until fragrant and translucent, about 3-4 minutes, filling your kitchen with a warm aroma.
Stir in the cooked quinoa, black beans, corn, diced tomatoes, and ground cumin. Cook everything together for another 5-7 minutes, stirring occasionally, until the mixture is heated through and fragrant with spices.
Use a spoon or piping bag to stuff each pepper generously with the filling, pressing down slightly to pack it in tightly.
Place the filled peppers upright in a baking dish. Cover the dish with aluminum foil to lock in moisture, and bake in the preheated oven for 25-30 minutes until the peppers are tender and slightly blistered.
Remove the foil and sprinkle vegan cheese or breadcrumbs on top if desired. Return to the oven and bake for another 5 minutes until the topping is golden and crispy.
Take the peppers out of the oven and let them rest for 5 minutes. This helps the juices settle and makes them easier to serve.
Garnish with chopped herbs and a squeeze of lemon if you like, then serve hot and enjoy the vibrant, smoky flavors of these hearty stuffed peppers.