From: Gooseberry Patch's Christmas Kitchen
Spread hashbrowns in a lightly greased 9"x9" baking pan. Layer ingredients as follows: half the onion, sausage, remaining onion, green pepper, mushrooms and cheese. In a separate bowl, mix together eggs, milk and seasonings. Pour egg mixture over top of casserole; cover with aluminum foil and refrigerate overnight. In the morning, bake, covered, at 350 degrees for 45 to 60 minutes. Uncover and bake an additional 15 minutes, or until golden and a knife inserted in center comes out clean. Serves 8 to 10.