In a greased 9x13 pan, combine water, milk, butter, and sauce mixes from the au gratin potatoes. Stir until sauce mixes are dissolved.

Stir in potatoes, sausage, and green beans. Top with cheddar cheese.

Bake, uncovered, at 400 degrees for 35 minutes (45 minutes if using frozen green beans), or until sauce is hot and bubbly. Let stand for 5 minutes before serving.