Combine the beef with soy sauce, sugar, sesame oil, garlic, and black pepper. Mix well and marinate for 20 minutes. The beef should be thin enough to cook in under 1 minute once it hits the pan. That genuinely thin slicing is what lets the marinade's own flavour penetrate quickly within its brief 20-minute window, since thicker strips would need considerably longer for the same seasoning to actually reach the meat's centre.

Beat the eggs with a small pinch of salt until smooth. Heat a lightly oiled nonstick pan over low to medium-low heat and cook the eggs into 1–2 thin omelettes, just set with no heavy browning. Cool slightly, roll, and slice into thin ribbons. Keep the egg tender and yellow; heavy browning makes the garnish look dull and slightly rubbery. That genuinely low, patient heat is what sets the egg into a smooth, tender sheet without ever browning it — real colour development here would work directly against the bright, clean yellow ribbons this garnish depends on.

Bring a pot of water to a boil. Blanch the spinach for 20–30 seconds, just until wilted. Drain, rinse briefly under cool water, then squeeze very well to remove excess moisture. Cut into shorter lengths if needed. This is critical: wet spinach will dilute the seasoning and make the noodles watery. That thorough, deliberate squeezing removes real, considerable moisture the spinach would otherwise release directly into the finished dish, protecting the seasoning's own concentration once everything is combined.

Stir together the soy sauce, sugar, sesame oil, garlic, and black pepper until the sugar is mostly dissolved. Set aside. Having the sauce fully mixed and ready before the noodles are even boiled is what lets it be tossed in immediately once the noodles drain, while they're still genuinely warm and receptive.

Boil the sweet potato noodles according to the package, usually about 6–7 minutes, until fully translucent and chewy-tender. Drain well. While still warm, toss the noodles with about two-thirds of the japchae sauce. If they are very long, cut them a few times with kitchen scissors. Do not overcook. The noodles should stay springy and elastic, not soft or swollen. That immediate tossing, while the noodles are still warm, is what lets them actually absorb the sauce evenly and stay properly separated — noodles left to cool and sit plain would clump together stubbornly, resisting the sauce once it's finally added.

Heat a little neutral oil in a skillet or wok over medium-high heat. Cook each component separately, removing it to a tray or bowl as soon as done: onion for 2–3 minutes until lightly softened but not collapsed; carrot for 1–2 minutes so it softens slightly but keeps some bite; shiitake mushrooms for 2–3 minutes until wilted and lightly browned; scallions for 20–30 seconds just to soften slightly. Cooking separately keeps the colors clean and prevents the vegetables from steaming each other. That genuine separation protects every vegetable's own distinct texture and colour — combined in one crowded pan, their collective released moisture would steam rather than sauté each component, dulling both the colours and the intended textural contrast.

In the same pan over high heat, cook the marinated beef quickly in a thin layer for 1–2 minutes until just cooked and lightly caramelized at the edges. If needed, do this in 2 batches so it sears instead of steams. Any juices that collect should be glossy and reduced, not watery. Cooking in properly small batches protects the pan's own high heat from dropping too far when the beef is added — a significant drop would let the meat release liquid and steam rather than actually searing and caramelizing at the edges.

Place the seasoned noodles in a large bowl. Add the cooked beef, spinach, onion, carrot, mushrooms, scallions, and egg ribbons. Add the remaining sauce and toss thoroughly with tongs or gloved hands until evenly mixed. Taste and adjust with a little extra soy sauce or a few drops more sesame oil only if needed. The final texture should be glossy and well-seasoned, but not wet or oily. Reserving the final third of the sauce specifically for this combining stage, rather than using it all on the noodles alone, ensures every other component gets its own real share of seasoning once everything is finally tossed together.

Transfer to a serving bowl and scatter generously with toasted sesame seeds. Serve warm or at room temperature. That flexibility in serving temperature reflects japchae's own real character — unlike many stir-fries, this dish holds its intended texture and flavour balance well even once it's cooled slightly from freshly cooked.