Go shopping for a recipe's main ingredient and come home with something else -- we've all been there. On the spot, you can figure out a suitable substitute on your own or consult with a grocery-department manager. For this recipe, flank steak became flatiron, because the flank steak was sold out. But the switch proved beneficial.
Both beef cuts are flat, quick-cooking and not especially tender. They come from different muscles of the animal (flank/belly vs. flatiron/shoulder). Typically, a rectangular piece of flat iron steak will weigh less than packaged flank steak and be easier to slice raw.
Slicing is what happens here as part of the prep, against the grain. Why that way? To shorten the meat's long fibers and create a more tender chew. With a flat iron steak, the process is simple: You only have to cut crosswise, with a long side parallel to the edge of your cutting board, and you will achieve success. The strips will be about an inch wide and not too long -- just right for threading onto skewers.
Treated to a fast and piquant marinade (for flavor), they go under the broiler or on a gas grill for just a few minutes.