Preheat the oven to 375°F (190°C).
Bring a large pot of water to a boil. Blanch the bell peppers by placing them in boiling water for 3 minutes. Remove and drain upside down on paper towels.
Heat 1 tablespoon of olive oil in a skillet over medium heat. Add the chopped onion and sauté for 3-4 minutes until translucent.
Add minced garlic to the skillet and cook for 1 minute until fragrant.
Add the diced tomato, dried Italian seasoning, salt, and black pepper. Cook for 3 minutes until the tomato softens.
In a mixing bowl, combine the ricotta cheese, cooked quinoa, sautéed onion and tomato mixture, mozzarella cheese, Parmesan cheese, and chopped parsley. Mix well.
Drizzle the remaining 1 tablespoon of olive oil inside each bell pepper.
Stuff each bell pepper with the ricotta mixture, pressing down gently to fill completely.
Place the stuffed peppers upright in a baking dish.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and the filling is golden on top.
Remove from oven and let cool for 5 minutes before serving.