Preheat your oven to 375°F.
Cut the tops off the bell peppers and remove the seeds and membranes.
In a mixing bowl, combine ricotta cheese, chopped spinach, cooked quinoa, garlic powder, Italian seasoning, salt, and black pepper. Mix until well combined.
Drizzle olive oil in a baking dish and spread 1/2 cup of marinara sauce on the bottom.
Stuff each bell pepper with the ricotta mixture and place them upright in the baking dish.
Top each stuffed pepper with the remaining marinara sauce and sprinkle shredded mozzarella cheese over the top.
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 cheese is bubbly and golden.
Let cool for a few minutes before serving.