Spread 1 side of each bread slice lightly with mustard. Layer ham and cheese between the mustard-spread sides of the bread slices. In a shallow bowl or pie plate beat together milk and egg white.
Coat an unheated nonstick griddle or large skillet with nonstick cooking spray. Preheat griddle over medium heat. Melt butter on griddle. Dip each sandwich in milk mixture, turning to coat. Place in skillet; cook for 1 to 2 minutes per side or until golden and cheese is melted. If desired, sprinkle with powdered sugar and serve with preserves. Makes 2 servings.