What kind of beef do you use for stir fry?
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Flank steak, cut across the grain. (Freeze it a bit to make it easier to cut.)
Yes, flank steak. I also use flap meat or flap steak if I can find it. It should be relatively inexpensive cut that you use.
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For a stir fry I would go with rib eye or skirt steak, observing the slight pre-freezing method recommended earlier. I do this when I grind meat as well.
I like to use rib eye.