Buy an unsliced loaf of rye bread in the deli section of your grocery store or bakery, and cut thick slices with a serrated or bread knife.
Stir together 1/2 cup mayonnaise and next 4 ingredients in a large bowl; stir in shredded cabbage, and set aside or chill until ready to serve.
Stir together mustard and remaining 1/4 cup mayonnaise. Spread evenly on 1 side of each bread slice. Layer evenly with 1 cheese slice and corned beef; top with remaining cheese slices. Place on a baking sheet.
Bake at 450° for 5 to 6 minutes or until cheese is melted. Spoon coleslaw mixture evenly over cheese, and serve immediately.