Cut the tops off the bell peppers and remove the seeds and membranes. Lightly brush the outside of each pepper with olive oil and place them upright in a baking dish.
In a large skillet, heat the olive oil over medium heat. Add the chopped red onion and sauté for 2-3 minutes until softened.
Add the minced garlic and cook for an additional 1 minute, making sure it does not burn.
Incorporate the chopped spinach into the skillet and cook until wilted, about 2-3 minutes.
Remove the skillet from the heat and stir in the cooked quinoa, crumbled feta cheese, dried oregano, black pepper, and salt to taste. Mix until well combined.
Carefully spoon the filling into each prepared bell pepper until they are well-stuffed.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
After 25 minutes, remove the foil and bake for an additional 10 minutes to allow the tops to slightly brown and the peppers to soften.
Once done, remove from the oven and let cool for a few minutes. Garnish with fresh parsley before serving.
Notes
Feel free to customize the filling with other vegetables or proteins.