Korean cooks slice beef for grilling very thinly to allow for quick marinating and only the briefest cooking. A sharp chef's knife is key to slicing the meat sufficiently thin. For easier slicing, first place the beef in the freezer just until firm, 20 to 30 minutes. Then, using a single deliberate motion, slice the meat across the grain 1/8 inch thick. Avoid sawing the meat, which results in slices with jagged edges. If the slices are too thick, place them between sheets of parchment or waxed paper and pound to an even 1/8-inch thickness with a meat pounder.