Check off each step as you cook.

1. 01Mix the seasoning.Stir soy sauce, sugar, sesame oil, garlic, and pepper until the sugar is mostly dissolved. Toss the beef with one-quarter of this mixture and refrigerate while preparing the vegetables. Keep the remaining seasoning separate from the raw meat.
2. 02Blanch the spinach and boil the noodles.Bring a large pot of water to a boil. Blanch spinach for about 1 minute, lift out, cool under running water, and squeeze dry. Boil the noodles in the same pot according to their packet, usually 6–8 minutes, until flexible and tender with a pleasant chew. Drain well, transfer to a large bowl, and shorten a little with kitchen scissors.
3. 03Cook the vegetables separately.Heat a little neutral oil in a broad skillet over medium-high heat. Cook onion for 3–4 minutes, carrot for 2–3 minutes, and mushrooms for 4–5 minutes, working in separate batches and adding oil as needed. Transfer each to the noodle bowl. Pull the drained spinach apart and add it too.
4. 04Cook the beef.Add the remaining neutral oil to the skillet. Cook the beef and its marinade for 3–5 minutes, turning frequently, until cooked through; a thick strip should reach at least 145°F / 63°C. Transfer to a clean plate and let stand for at least 3 minutes while dressing the noodles.
5. 05Toss and serve.Pour the reserved seasoning over the noodles and vegetables and toss thoroughly with tongs. Add the rested beef and sesame seeds. Return to the skillet over medium heat for 1–2 minutes if needed to warm through, then serve.

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