Prep the beef: Slice the steak thinly against the grain. Toss with soy sauce, cornstarch, and sesame oil. Let it sit 10–15 minutes while you prep everything else.
Cook the noodles: Boil according to package directions until just shy of tender. Drain and rinse briefly to stop cooking. Toss with a few drops of oil to prevent sticking.
Stir together the sauce: In a small bowl, whisk hoisin, soy sauce, oyster sauce, rice vinegar, brown sugar, red pepper flakes, water, and cornstarch until smooth.
Heat the pan: Set a large skillet or wok over medium-high heat. Add the oil and let it shimmer.
Sear the beef: Spread the beef in a single layer. Cook 60–90 seconds per side until browned and mostly cooked through. Remove to a plate.
Sauté aromatics and veg: Add a touch more oil if needed. Stir-fry the garlic, ginger, and green onion whites for 20–30 seconds, just until fragrant. Add the vegetables and cook 2–3 minutes until crisp-tender.
Bring it together: Return the beef to the pan. Pour in the sauce. Toss and let it bubble 1–2 minutes until glossy and thickened.
Add noodles: Add the cooked noodles and green onion tops. Toss with tongs until everything is coated. Drizzle with toasted sesame oil. Taste and adjust with a splash of soy or a squeeze of lime.
Serve: Top with sesame seeds and extra scallions. Eat hot, straight from the pan.
