Eastern European flavors are celebrated in this quick and easy casserole version of a favorite sandwich. History isn't clear as to how the Reuben sandwich got its name. One source says the sandwich was named in 1914 for the owner of New York's once-famous Reuben's delicatessen. But some say Reuben Kay, a wholesale grocer from Omaha, named the sandwich in 1955. Any kind of mustard can be used in this dish. Varieties to try include stone-ground, Dijon, spicy, sweet-hot and honey mustard.