Our starting point for this dish was a recipe from Charles Phan of The Slanted Door restaurant in San Francisco. Phan calls for flank steak in his re-creation of a beef stir-fry with noodles that he had in Hanoi. We, however, like the rich, meaty flavor of sirloin tips, a cut sometimes sold as flap meat or faux hanger. It's fine if the tips are in two or three pieces, as the meat will be sliced for serving. If you can't find sirloin tips, opt for flank steak. If the flank is of an even ½-inch thickness, it does not need to be pounded. We used Thai Kitchen thin rice noodles (sometimes called rice vermicelli). Check the package directions for soaking times, as they can vary. And if the noodles are in very long strands, snip them a few times with scissors after draining to make them easier to eat. If the taste of the fish sauce is too strong in the dressing, thin it with a few teaspoons of water.