1Spray 12-inch skillet with cooking spray; heat over medium-high heat. Add onion; cook 3 to 5 minutes, stirring frequently, until softened. Remove onion from skillet.
2Spread 4 bread slices with dressing. Top each slice with one-fourth of the onion, 2 strips bell pepper, 2 tablespoons sauerkraut and 1 cheese slice. Top with remaining bread slices.
3Heat same skillet over medium-low heat. Spray outsides of sandwiches with cooking spray; place sandwiches in skillet. Cook 4 to 5 minutes, turning after 2 minutes, until cheese is melted and bread is toasted.