Depending on the cut you go with, you might want to tenderize it a bit before cooking. Salting your beef is a good start so salt the entire piece before slicing it—but make sure to let it sit for at least 40 minutes before cooking. The other way to ensure that you have tender beef for your stir fry is by cutting it against the grain. Hold your knife at a 45-degree angle and slice thin strips to make the muscle fibers shorter, making the beef less chewy.