Whatever type of steak you choose to use, there are a few simple steps you can follow to ensure that your beef is perfectly prepared for a stir-fry. First, trim away any excess fat from the meat. Next, slice the beef into thin strips, no more than 0.5cm (quarter-inch) thick. If the strips are too fat, they will take longer to cook and may end up tough and chewy.
When you come to slice the beef, you can opt to place it in the freezer for 20-30 minutes beforehand. This will make it easier to slice. When slicing always cut across the grain. This means slicing so your knife goes through all the long fibres running down the meat, not parallel to them. This is particularly critical if you are using flank steak, which can be very chewy when sliced with the grain.