Rich, tangy, meaty mix of corned beef, sauerkraut, and melted cheese, makes a satisfying meal. Enjoy!
Position a rack 6 inches from the broiler and another rack in the bottom third of the oven. Heat the broiler on high.
Combine the mayonnaise, ketchup, and relish in a small bowl.
Cut two 12x18-inch sheets of foil. Reserve 1 Tbs. of the liquid from the sauerkraut and then rinse and drain it. Squeeze the sauerkraut with your hands to remove as much liquid as possible and put it on 1 piece of foil. Put the corned beef on the other and sprinkle with the reserved sauerkraut liquid. Draw up the sides and seal each to make a package. Put both packages on a rimmed baking sheet and bake on the bottom rack of the oven until warmed through, about 5 minutes.
Meanwhile, put the bread on another rimmed baking sheet and broil until well toasted, about 1 minute per side. Butter one side of the toasted bread and arrange butter side down on the baking sheet. Spread each piece with 1 Tbs. of the dressing.
Divide the corned beef among the bread slices. Top with the sauerkraut and the cheese. Broil on the lower rack until the cheese begins to bubble, about 3 minutes. Move the baking sheet to the upper rack and broil until the cheese browns lightly, 1 to 2 minutes. Serve immediately.