Spread one side of each bread slice lightly with mustard. On two of the bread slices layer the Jarlsberg cheese, prosciutto, and melon on top of mustard. Spread a thin layer of sour cream over melon slices. Add salami, Gouda, and romaine, if desired. Top with remaining two bread slices, mustard side down. Wrap in plastic wrap. Chill for 1 to 6 hours. Makes 2 sandwiches.