Heat oven to 350°F. In 2-quart saucepan, heat water, butter and salt to boiling. Remove from heat. Stir in milk and dry potatoes just until moistened. Let stand about 30 seconds or until liquid is absorbed; whip with fork until fluffy. Stir in relish, mayonnaise, onion and mustard. Spread in ungreased 1-quart casserole.