Preheat oven to 350 degrees F.
In a large mixing bowl, combine thawed hash brown potatoes, melted butter, Vegan Condensed Cream of Mushroom Soup, sour cream, chopped onion, Cheddar cheese, Parmesan cheese, sea salt, and ground black pepper.
Into a prepared 13x9-inch baking dish, spread the hash brown mixture in an even layer.
Cover and bake 45 minutes.
Uncover and top with French Fried Onions.
Return casserole to the oven. Continue baking, uncovered, for an additional 10 minutes and serve hot.