Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Add the bell peppers and blanch for 3 minutes to soften slightly. Remove and drain upside down on paper towels.
In a medium skillet, heat olive oil over medium heat. Add the chopped onion and sauté for 3 minutes until translucent.
Add the minced garlic and cook for another 1 minute until fragrant. Remove from heat.
In a large bowl, combine ricotta cheese, chopped spinach, grated Parmesan, cooked quinoa, sautéed onion and garlic, dried Italian seasoning, salt, and black pepper. Mix well until fully incorporated.
Place the bell peppers upright in a baking dish. Spoon the ricotta mixture evenly into each bell pepper, filling them generously.
Top each stuffed pepper with a tablespoon of marinara sauce, then sprinkle shredded mozzarella cheese on top.
Cover the baking dish with foil and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and golden.
Remove from oven and let cool for 5 minutes before serving.