Preparations

Hold a sharp Chinese cleaver or chef’s knife diagonal to the board and slice the beef against the grain into thin slices evenly ⅛ inch thick. (Holding the knife on a diagonal broadens the slices.) Cut the slices crosswise into pieces about 2 inches long, then spank lightly with the broad side of the knife to tenderize and even them.

Combine the soy, sugar, cornstarch, and oil, whisking until smooth. Toss well with the beef, using your fingers to coat and separate the slices. Seal airtight, then put aside to marinate 1 hour at room temperature or several hours to overnight in the refrigerator. Stir once or twice while marinating to redistribute the seasonings. Bring to room temperature before frying.

Cut the leeks crosswise into 2-inch lengths, then lengthwise into shreds ¼ inch wide. Rinse thoroughly, then shake well to remove excess water. Shredded, the leeks may be bagged airtight in plastic and refrigerated overnight.

Blend the sauce ingredients thoroughly to mix, and leave the spoon in the bowl.

Deep-frying the beef

Have the beef, cooking chopsticks or a wooden spoon, and a large Chinese mesh spoon or a metal colander set securely over a pot all within arm’s reach of your stovetop. Stir the beef to loosen the slices.

About 10–15 minutes before serving (or 20–25 minutes if making the pot-browned noodles), heat a wok or deep, heavy skillet over high heat until hot. Add the oil and heat to the light-haze stage, 350° on a deep-fry thermometer. Adjust the heat so the temperature does not rise, then test the oil with one slice of beef. It should fry slowly, not gustily.

Give the beef a stir, then slide it into the oil. It will disappear under a crown of white bubbles. Stir slowly to separate the slices. After 15 seconds, remove the meat in one quick motion with the mesh spoon, or drain the oil swiftly into the pot with the metal colander. Work quickly, lest the beef overcook.

Reserve the frying oil. What remains after stir-frying may be strained through cheesecloth, bottled, and refrigerated for future use.

(At this point begin frying the noodles with the reserved oil, if you are making Pot-Browned Noodles. Proceed to the next stir-frying step as soon as you have flipped the noodles over.)

Stir-frying the beef and leeks

Have the leeks, salt, sugar, deep-fried beef, and sauce ingredients all within easy reach.

Wipe the wok or skillet clean and return it to high heat until hot enough to evaporate a bead of water. Add 2 tablespoons of the reserved oil and swirl to glaze the pan. When the oil is hot enough to sizzle a leek shred, add the leeks to the pan. Stir-fry briskly to glaze the shreds with oil, about 20–30 seconds, adjusting the heat so they sizzle without scorching. Sprinkle with the salt and sugar, then toss another 30–60 seconds until they are soft.

Add the meat and toss briskly to combine. Stir the sauce ingredients, scrape them into the pan, and stir to mix. Raise the heat for 5–10 seconds to bring the liquids to a boil, stirring briskly all the while.

Remove the mixture to a heated serving platter of contrasting color. Arrange a few green leeks on top for a pretty effect, then serve at once. (Or, if you are using the beef to crown the noodles, turn off the heat and cover the pot briefly until the noodles are ready.)

Leftovers keep 2–3 days, refrigerated, and are tasty at room temperature.