In a medium saucepan, bring vegetable broth to a boil. Add quinoa, cover, and simmer for 12-15 minutes until fluffy. Fluff with a fork.

Heat olive oil in a large skillet over medium heat. Add onion and garlic, and sauté until fragrant. Add bell pepper, zucchini, mushrooms, and cherry tomatoes. Cook for 5-7 minutes until softened. Stir in baby spinach and seasonings until spinach wilts.

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

Preheat the oven to 190°C (375°F). In a large mixing bowl, combine cooked quinoa, sautéed vegetables, and sauce. Mix and transfer to a greased baking dish.

Sprinkle mozzarella and Parmesan cheese on top. Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 10 minutes until golden.

Let casserole rest for 5 minutes, garnish with fresh herbs, and enjoy!