In a medium saucepan, bring 2 cups of vegetable broth to a rolling boil. Once boiling, add 1 cup of rinsed quinoa, cover tightly, and reduce the heat to low. Let it simmer for 12-15 minutes, or until the liquid is fully absorbed. When done, fluff the quinoa with a fork and set it aside.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add 1 chopped onion and 2 minced garlic cloves, cooking until fragrant, about 2 minutes. Stir in 1 chopped bell pepper, 1 diced zucchini, and 1 cup of sliced mushrooms. Sauté all the veggies for 5-7 minutes until they are tender. Finally, fold in 1 cup halved cherry tomatoes and 2 cups baby spinach, cooking until the spinach wilts, then remove from heat.

In a mixing bowl, combine 1 cup of tomato sauce, ½ cup of Greek yogurt, and a pinch of red pepper flakes if desired. Stir thoroughly until the sauce is creamy and well-blended.

Preheat your oven to 190°C (375°F). In a large bowl, combine the cooked quinoa, sautéed vegetables, and the creamy sauce. Transfer the mixture into a greased 9x13-inch baking dish.

Sprinkle 1 cup of shredded mozzarella cheese evenly over the top of the casserole. Cover with foil and bake for 20 minutes, then remove the foil and bake for an additional 10 minutes until the cheese is bubbly and golden brown.

Allow your casserole to rest for about 5 minutes before serving. Garnish with freshly chopped parsley or basil.