Corned Beef Reuben with Savoy Cabbage Slaw

Name: Corned Beef Reuben with Savoy Cabbage Slaw
Ingredients: 1 cup very thinly sliced savoy cabbage (you can substitute green cabbage); 1 tablespoon rice vinegar or white wine vinegar; 1/2 teaspoon sugar; 1/4 teaspoon celery seeds; Kosher salt and freshly ground black pepper; 1/4 cup mayonnaise; 2 teaspoons ketchup; 2 teaspoons drained sweet pickle relish; 1/4 teaspoon paprika; 8 slices rye or marbled rye sandwich bread; 8 thick slices Swiss cheese; 1 pound leftover corned beef, coarsely chopped; 3 tablespoons unsalted butter, at room temperature