Mushroom & Onion Reubens

Spray a 10-inch nonstick skillet with nonstick cooking spray. Add mushrooms and onion. Cook and stir over medium-high heat until mushrooms are tender and onions are slightly browned. Reduce heat to low. Add sauerkraut. Cook 3 to 4 minutes or until sauerkraut is thoroughly heated, stirring occasionally. Place bread slices on an ungreased cookie sheet. Broil 4 to 6 inches from heat for 1 to 2 minutes or until bread is toasted on one side. Spread mustard on toasted side of 4 bread slices. Top each with one-fourth of the sauerkraut mixture, 1 slice of cheese, and a bread slice, toasted side down. Return sandwiches to cookie sheet. Broil an additional 1 to 2 minutes each or until bread is crisp and cheese is melted.