Preheat the oven to 190°C (375°F).
Cut the tops off bell peppers and remove seeds and membranes. Set aside.
In a pan, sauté onions, garlic, and chopped vegetables with a little olive oil until softened.
Add cooked quinoa and lean protein (like ground turkey, chicken, or lentils) to the pan and mix well. Season with salt, black pepper, and herbs of your choice.
Spoon the filling into the hollowed bell peppers, pressing gently to pack.
Place stuffed peppers in a baking dish, cover with foil, and bake for 25–30 minutes until peppers are tender.
Optional: Uncover, sprinkle with a small amount of cheese, and bake for an additional 5 minutes until lightly golden.