Preheat oven to 375 degrees.
Lightly grease a 9 x 13 glass baking dish.
Mix the hash browns and butter together and press the mixture along the bottom and sides of the dish to createa crust.
In a pan, brown the sausage and onions. Drain any excess grease.
In a large mixing bowl, combine the cottage cheese, cheddar cheese, mushrooms, and eggs with the cooked sausage and onions.
Pour the mixture over the hash browns.
Bake for 1 hour.