In a medium saucepan, bring vegetable broth to a boil. Add quinoa, cover, and simmer for 12-15 minutes until liquid is absorbed. Fluff and set aside.

In a large skillet, heat olive oil over medium heat. Sauté onion and garlic until fragrant, about 3 minutes. Add bell pepper, zucchini, and mushrooms; cook for 5-7 minutes until softened. Stir in cherry tomatoes and spinach until wilted.

In a mixing bowl, combine tomato sauce, Greek yogurt, and red pepper flakes. Stir until creamy and well blended.

Preheat your oven to 190°C (375°F).

In a large mixing bowl, combine cooked quinoa, sautéed vegetables, and prepared sauce. Mix until well coated. Transfer to a greased baking dish, spreading evenly.

Sprinkle mozzarella and parmesan cheese over the top. Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 10 minutes until cheese is bubbly.

Let the casserole rest for 5 minutes. Garnish with fresh parsley or basil and serve warm.