You probably already have most of the ingredients for this recipe's marinade in the house. If you can just manage to find yourself a skirt steak, you will really be in business. You may have loved skirt steak and not even known it when it was served to you at a restaurant and called fajitas. Now it's becoming much more readily available to the home cook. Skirt steak is long and thing and has great beef flavor. It is perfect for grilling or a quick sear in the pan. Tough but flavorful (like flank steak and hanger steak), skirt steak should never be cooked beyond medium-rare. Let it rest for a few minutes before you carve it, then slice it very thin and at an angle. If you don't find skirt steak at your supermarket, ask the butcher to get it for you. (You'll be surprised at how much power you can wield as a consumer.) Cellophane noodles, also called bean thread noodles, are made from the starch of mung beans. When they are soaked, they become soft, much like cooked pasta, and when quickly fried, as they are here, they puff up into crispy white strands. They absorb the flavors of the foods they are served with. Here they provide a dramatic-looking backdrop to the beef. If you can't find them, serve the beef on plain rice or on the boiled noodles of your choice.
Combine the soy sauce, vinegar, scallions, sugar, ginger, sesame oil, and pepper flakes in a bowl. Whisk well to blend. Reserve 1/4 cup and set aside. Pour the rest into a plastic bag with a resealable closure. Add the skirt steak and seal. Marinate in the refrigerator, turning the bag often, for at least 4 hours or overnight.
Fill a large deep saucepan to a depth of about 2 inches with vegetable oil. Heat over medium-high heat until a deep-fat thermometer reads 375 degrees F. (Alternatively, use an electric deep-fat fryer) Pull apart the clusters of noodles into clumps of about 10 strands each. Working in batches, fry the noodles, turning once, until they puff up and turn white, about 10 seconds. Use a slotted spoon or a spider (a slotted spoon designed like a spider's web used for deep-frying) to transfer to paper towels to drain.
Remove the skirt steak from the marinade and pat dry with paper towels. (The drier the beef, the better it will sear.) Working in batches, heat half the oil in a large skillet over high heat until almost smoking. Season the meat with salt and pepper. Place in the skillet and reduce the heat to medium-high. Cook until seared on the outside and rare to medium-rare on the inside, 2 to 3 minutes per side. Transfer to a platter, cover loosely with foil, and let rest for 5 minutes. Repeat with the remaining oil and skirt steak. Cut into very thin slices, holding the knife at an angle and cutting against the grain. Toss with the reserved 1/4 cup sauce in a large bowl until well coated.
Crumble the fried noodles and mound on individual serving plates. Top with equal portions of the beef and garnish with chopped scallions.
Recipe courtesy of Sara Moulton, Sara Moulton Cooks at Home, Broadway Books, 2002