Preheat the oven to 375°F.
Cut the tops off the bell peppers and remove the seeds and membranes. Drizzle the outside of the peppers with olive oil and place them in a baking dish.
In a mixing bowl, combine the ricotta cheese, cooked quinoa, chopped spinach, marinara sauce, half of the mozzarella cheese, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
Spoon the ricotta mixture into each bell pepper, packing it down gently.
Sprinkle the remaining mozzarella cheese on top of the stuffed peppers.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the peppers are tender and the cheese is bubbly and golden.
Let the peppers cool for a few minutes before serving.