Spread rectangle with dressing to within 1/4 inch of edges. Top with corned beef, cheese and sauerkraut. Press remaining 2 rectangles to 7x5 inches, firmly pressing perforations to seal. Spread each with 2 teaspoons mustard. Place each rectangle mustard-side down on sauerkraut, pinching center seam to seal. Brush top with water, and sprinkle with caraway seed.