Prep everything first (mise en place). Slice the beef thin across the grain into 3–4 cm strips if not already sliced — cutting across the grain shortens the muscle fibers and keeps each bite tender. Halve and rinse the baby bok choy, grate the ginger, mince the garlic, and slice the scallion. Having all ingredients ready keeps the fast stir-fry from overcooking.

Marinate the beef: In a small bowl combine 1 tbsp light soy sauce, 1 tsp cornstarch and about 1/2 of the grated ginger (reserve the rest for the sauce). The cornstarch gives the beef a thin protective coating that helps it brown quickly while staying juicy (a simple 'velveting' technique). Let the beef sit 8–10 minutes while you prepare noodles and bok choy.

Cook the udon: If using fresh udon, loosen it briefly in boiling water for 30–45 seconds, drain and toss with a drizzle (1 tsp) of sesame oil to prevent sticking. If using dry udon, cook to just al dente according to package directions (usually 7–8 minutes), drain and rinse under warm water. Par-cooking the noodles keeps them springy and prevents them from absorbing too much sauce in the wok.

Heat a large heavy skillet or wok over high heat until very hot — you want a surface hot enough to sear but not smoke excessively. Add the neutral oil and swirl. High heat gives a quick Maillard sear on the beef for savory flavor; working hot and fast prevents the beef from stewing and becoming tough.

Sear the beef in a single layer: Shake off excess marinade and add the beef in a single layer. Let it sear undisturbed for 30–45 seconds to develop color, then stir or toss for another 30–45 seconds until mostly cooked but still slightly pink inside (it will finish in the sauce). Remove the beef to a plate — you want it slightly underdone now so it stays tender later.

Quickly stir-fry aromatics and bok choy: Return the pan to high heat if it cooled. Add 1 tsp sesame oil, the minced garlic, the remaining grated ginger, and the white parts of the scallion first; stir 10–15 seconds until fragrant (blooming the aromatics releases bright ginger-garlic flavor). Add the bok choy cut side down and sear for 30–45 seconds, then toss — this quick hit of heat keeps the stems crisp-tender while giving a bit of char on the leaves and cut surfaces.

Build the sauce: In a small bowl whisk remaining 1 tbsp light soy sauce with 1 tbsp mirin (or dry sherry) and 1 tsp water. Pour the sauce into the wok and stir to deglaze — the liquid will lift browned bits from the pan and carry that flavor into the sauce. Let it bubble briefly to marry flavors and reduce slightly.

Return beef and add noodles: Add the seared beef back into the wok along with the udon noodles. Toss vigorously to coat everything with the sauce. Tossing over high heat warms the noodles through quickly and helps the sauce cling to the strands; the residual cornstarch on the beef will also give the sauce a silky sheen.

Finish and season: Drizzle the 1 tbsp sesame oil over the pan, add most of the sliced scallion greens (reserve a few for garnish), and toss once more. Taste and season with a pinch of salt or a dash more soy if needed, and freshly ground black pepper. The sesame oil at the end provides a fragrant finish; adding it too early would let it burn and lose aroma.

Plate and garnish: Divide between two bowls, scatter the toasted sesame seeds and remaining scallion, and serve immediately. Prompt plating keeps the bok choy crisp and the noodles at their best texture.