MIX salad dressing and flour; gently whisk in milk until smooth.

MICROWAVE on HIGH for 2-3 minutes (stirring once during cooking) or on Medium heat on stove (stirring continuously) until sauce is thick.

STIR in cheese until smooth and creamy.

PLACE 1/2 cup sauce on bottom of lightly greased 9 inch baking dish.

LAYER with potatoes, ham, and onion; pour remaining sauce over and cover with foil.

BAKE at 350°F for 20 minutes; remove foil and bake 10 minutes longer or until lightly browned.