For dressing, whisk first 4 ingredients until sugar is dissolved. Cook potatoes according to package directions.
Meanwhile, place a skillet coated with cooking spray over medium heat. Add steak; cook until meat reaches desired doneness (for medium-rare, a thermometer should read 135°; medium, 140°), 5-6 minutes per side. Remove from heat; let stand 5 minutes before slicing.
To serve, divide lettuce, tomato and onion between 2 plates; top with potatoes, steak and cheese. Drizzle with dressing.