Combine first 4 ingredients; rub mixture over steak. Place a nonstick skillet over medium-high heat until hot. Coat steak and onion slices with cooking spray; add steak and onion to skillet. Cook 12 minutes or to desired degree of doneness, stirring onion often and turning steak once.
Combine mayonnaise and mustard, stirring well; spread over 1 side of each bread slice. Thinly slice steak across the grain. Arrange steak evenly on 4 bread slices; top evenly with onion. Top with remaining bread slices.