Cook the quinoa: Rinse the quinoa under cool water, until the water comes out transparent. Put rinsed quinoa in a medium saucepan. Add 1 cup (240ml) of water, 1/2 teaspoon salt and bring to a boil over high heat. Reduce heat to low, cover, and simmer for 15 minutes until the quinoa is fluffy. Remove from heat and let stand, covered, for 5 minutes. Set aside.
Preheat oven to 400F (200C).
In a large pan or skillet heat olive oil. Add chopped onion and sauté for 5-6 minutes. Add minced garlic and cook for 2 minutes more.
Add cooked quinoa, black beans, crushed tomatoes, seasoning, chopped parsley, and cook for 5 minutes.
Cut the tops of the bell peppers and remove the seeds. Fill loosely the peppers with the filling mixture.
Spread few tablespoons of tomato sauce at the bottom of a 9X13-inch (22X33cm) square baking dish or a 30cm round baking dish. Arrange stuffed peppers in the baking dish.
Sprinkle some shredded cheese on top.
Grease with oil a piece of foil and cover the pepper with oiled side down.
Bake, covered, for 35-40 minutes, remove the foil and bake for 10 minutes more until the peppers are tender and the cheese is melted.