Preheat oven to 350°F / 177°C / Gas Mark 4
Cut peppers in half, remove seeds and membranes.
Place peppers cut side down on baking sheet. Cover tightly with aluminum foil and bake for 20 minutes.
While peppers are baking, start the filling - On medium heat, add olive oil to a medium-large pot.
Add diced onion and sauté until beginning to soften, about 6 - 7 minutes. Add banana pepper (if using) and garlic, stir to combine.
Add diced tomatoes, dried basil, dried oregano, salt and pepper. Stir to combine. Taste and adjust seasoning if needed.
Fold in quinoa, beans, parsley, fresh basil and leftover veggies, if using. Set aside.
Remove peppers from oven, carefully uncover and flip over. When peppers are cool enough to handle, fill with quinoa and bean mixture.
Top with grated mozzarella and bake for an additional 20 minutes, or until cheese is melted and browned, and peppers are warmed through.