Rinse the quinoa under cold water. In a large saucepan, combine 1 cup of rinsed quinoa with 2 cups of water. Bring it to a boil, then reduce the heat to low, cover, and simmer for about 15 minutes or until the quinoa is fluffy and all water is absorbed. Remove from heat and let it sit covered for 5 minutes.
In the same saucepan, heat a drizzle of olive oil over medium heat. Add the diced onion and bell pepper, sautéing for about 5 minutes until softened. Then, add the sliced mushrooms and minced garlic, cooking for another 3-4 minutes until the mushrooms release their moisture and begin to brown.
Stir in the fresh baby spinach and diced roma tomatoes, cooking until the spinach wilts. Add the cooked quinoa to the vegetable mixture, followed by the tomato sauce and Italian seasoning. Mix well until all ingredients are thoroughly combined.
Preheat your oven to 375°F (190°C). Transfer the quinoa and vegetable mixture into a greased 9x13 inch baking dish. Evenly sprinkle the vegan mozzarella shreds over the top.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Once out of the oven, let the casserole cool for a few minutes. Top with fresh basil before serving for an added burst of flavor.
