Preheat oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a large bowl, combine the thawed spinach, cream of mushroom soup, sour cream, Parmesan cheese, cheddar cheese, onion, melted butter, garlic powder, salt, and pepper.
Pour the mixture into the prepared baking dish and spread evenly.
Bake for 30 minutes, or until golden brown and bubbly.
Let cool slightly before serving.
Notes
Enjoy this casserole as a side dish or as a vegetarian main course!