Bring 2 cups of vegetable broth to a boil in a saucepan. Rinse 1 cup of quinoa thoroughly, then add to boiling broth. Simmer uncovered for 12-15 minutes until liquid is absorbed, then fluff with a fork.

In a skillet, heat 2 tablespoons of olive oil. Add 1 diced onion and 2 minced garlic cloves, sauté for 2-3 minutes until fragrant. Add 1 diced bell pepper, 1 chopped zucchini, and 1 cup of sliced mushrooms, cooking for about 5-7 minutes until softened. Stir in 1 cup halved cherry tomatoes and 2 cups baby spinach until spinach wilts.

In a bowl, combine 1 cup of tomato sauce, ½ cup of Greek yogurt, and red pepper flakes. Mix until creamy.

Preheat oven to 190°C (375°F). In a mixing bowl, combine the fluffed quinoa, sautéed vegetables, and sauce. Transfer to a greased 9x13-inch baking dish.

Sprinkle 1 cup of shredded mozzarella and ¼ cup of grated Parmesan over the mixture. Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 10 minutes until cheese is golden.

Let the casserole rest for 5 minutes before serving. Garnish with fresh parsley or basil.