Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with olive oil or non-stick cooking spray.
Cook Quinoa:
In a medium saucepan, bring the vegetable broth or water to a boil. Add the rinsed quinoa, reduce the heat to low, cover, and simmer for about 15 minutes, or until the quinoa is cooked and fluffy. Remove from heat and fluff with a fork.
Sauté Veggies:
In a large skillet, heat olive oil over medium heat. Add the diced onion and sauté until translucent, about 3-4 minutes. Add the minced garlic and cook for another 1-2 minutes until fragrant.
Add the diced bell pepper and zucchini to the skillet. Cook, stirring occasionally, for about 5 minutes, or until the vegetables are slightly softened.
Stir in the cherry tomatoes, chopped spinach, dried oregano, dried basil, crushed red pepper flakes (if using), salt, and pepper. Cook for an additional 2-3 minutes until the spinach wilts and the tomatoes are softened.
Assemble Casserole:
In the prepared baking dish, spread half of the cooked quinoa evenly on the bottom. Spoon half of the pizza sauce over the quinoa layer, followed by half of the sautéed vegetable mixture. Sprinkle half of the shredded mozzarella cheese on top.
Repeat the layers with the remaining quinoa, pizza sauce, vegetable mixture, and shredded mozzarella cheese.
Bake:
