This delicious and healthier version of a cheesesteak sandwich features a whole grain bun, reduced-fat blue cheese dressing for a tangy kick, and sweet caramelized onions for a perfect balance of flavors in this wholesome twist on a classic favorite.
Combine the yogurt, mayonnaise, and blue cheese. Set aside.
To make caramelized onions, spray a hot skillet with cooking spray. Add onions and cook over medium low heat. Cook until onions have a translucent caramelized color, about 20 minutes. Add brown sugar and pepper to taste.
Heat a cast iron skillet until hot. Season flank steak with salt and pepper. Place steak in hot skillet and turn heat down to medium. Cook 7-8 minutes per side or until desired doneness. Remove from skillet onto a cutting board and tent with foil. Let sit at least 5 minutes before slicing.
Toast bun in skillet if desired, slice steak into thin slices. Layer sandwich as follows: spinach, tomato, beef slices, caramelized onions, blue cheese dressing.