Preheat oven to 375 F and lightly grease a 23 centimeter square dish.
Heat a large non-stick skillet over medium high heat and add in the olive oil. Add the onion and cook for three to four minutes until softened, then add in the garlic and oregano. Sauté for 30 seconds.
Add in the bell peppers and mushrooms and cook until softened and lightly browned.
Transfer the cooked vegetables to a large bowl and add in the olives, quinoa, sauce, one cup of mozzarella, parmesan and salt and pepper, to taste.
Pack into the baking dish and top with the remaining 1/2 cup mozzarella cheese. Bake for 20-25 minutes until golden brown around the edges and bubbling. Top with basil leaves, if desired, as garnish.