To butterfly the flank steak, start with the meat grain running up and down in front of you. With your knife, begin cutting the meat in half. Essentially, you want a thinner, larger flank steak. Continue cutting the flank steak in half until you reach about 1 inch from completely cutting through. Lay the cut flank steak open and even out the middle so there is not a thicker piece in the center.
Spread the flank steak out so that the grain is running left to right. Begin layering the flank steak with the spinach and feta cheese. Begin rolling the flank steak tightly and away from you. Evenly insert the skewers 1 to 2 inches along the seam of the pinwheel to hold it together on the grill.
Grill the pinwheels on each side for about 3 to 4 minutes or until your desired doneness. Let rest for about 5 minutes before slicing. Sprinkle salt and pepper over the slices.