01 - Preheat oven to 375°F. Grease a 9x9-inch baking dish.
02 - Combine quinoa and vegetable broth in a medium saucepan. Bring to a boil, reduce heat, cover, and simmer for 15 minutes until liquid is absorbed. Fluff with a fork and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add chopped onion and cook for 3 minutes until translucent.
04 - Add minced garlic and sliced mushrooms to the skillet; cook for 6–7 minutes until mushrooms are tender and moisture evaporates. Stir in chopped spinach and cook until wilted, about 1 minute. Remove from heat.
05 - In a separate saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute. Gradually add milk and cream, whisking constantly until thickened, 3–4 minutes.
06 - Remove from heat and stir in grated Parmesan cheese, nutmeg, salt, and black pepper until sauce is smooth.
07 - In a large mixing bowl, thoroughly combine cooked quinoa, sautéed vegetables, and Alfredo-style sauce.
08 - Transfer mixture to prepared baking dish. Evenly sprinkle mozzarella cheese and additional Parmesan on top.
09 - Bake for 20–25 minutes until the casserole is golden and bubbling.
10 - Allow to rest for 5 minutes before serving. Garnish with chopped fresh parsley if desired.