Make the Sandwiches Spread a thin coat of mustard on each piece of bread. Place a few slices of ham and a generous sprinkle of Gruyère on each of 4 bread slices. (Reserve about one-quarter of the cheese.) Top with a second slice of bread, mustard side down. Place sandwiches on a baking sheet. Spoon a generous amount of white sauce onto each sandwich. Divide the remaining Gruyère over the top.