Heat oven to 350B Grease 8Nx 12Npan.
In large bowl, cover potato slices with 3 c hot water; let stand while frying sausage and onion (fry sausage until meat is crumbly and loses pink color); drain.
Drain potatoes and add with green pepper and mushrooms to sausage mixture.
Spoon into pan.
In bowl, combine au gratin seasoning envelope with 1/2 c water.
Beat in milk and eggs and mix well.
Pour over sausage mixture.
Bake 35-40 min To serve, cut into squares.