In a medium saucepan, bring vegetable broth to a boil and stir in quinoa. Reduce heat and simmer for 12-15 minutes.

Heat olive oil in a skillet over medium heat, add onion and garlic, and sauté for 2 minutes. Add remaining vegetables and cook for 5-7 minutes.

In a bowl, combine tomato sauce, Greek yogurt, and red pepper flakes until smooth.

Preheat oven to 190°C (375°F). Mix quinoa, sautéed vegetables, and sauce in a bowl and transfer to a greased baking dish.

Top with mozzarella and Parmesan cheese, cover with foil, and bake for 20 minutes. Remove foil and bake another 10 minutes.

Let casserole rest for 5 minutes, then garnish with parsley or basil before serving.