Cut the tops off the bell peppers and remove the seeds and membranes. Lightly brush the outside with olive oil and place them upright in a baking dish.
In a skillet, heat olive oil over medium heat. Add the minced garlic and sauté until fragrant, about 1 minute.
Add the chopped spinach to the skillet and cook until wilted, about 3-4 minutes. Season with salt, pepper, and red pepper flakes.
In a large mixing bowl, combine the cooked spinach, ricotta cheese, mozzarella cheese, Parmesan cheese, and cooked quinoa or rice (if using). Mix until well combined.
Stuff each bell pepper generously with the spinach and ricotta mixture, pressing down slightly to pack it in.
Cover the baking dish with foil and bake for 25 minutes. Then, remove the foil and bake for an additional 10-15 minutes, until the peppers are tender and the tops are golden.
Remove from the oven and let cool for a few minutes before serving. Garnish with fresh basil or parsley.
Notes
Feel free to adjust the spice level with red pepper flakes.