Soak the Dried Shiitake Mushrooms in hot water for 20–30 minutes until softened; reserve ½ cup of the soaking liquid, then squeeze and slice the mushrooms.
Bring a large pot of water to a boil and cook the Dangmyeon until tender but still chewy (follow package times, usually 6–8 minutes). Drain, rinse under cold water, cut into shorter lengths and toss with a bit of the reserved mushroom liquid to keep them glossy.
Whisk together 4 tbsp Soy Sauce, 2 tbsp Sugar, 1 tbsp Sesame Oil, 2 minced cloves Garlic and a generous grind of Black Pepper to make the japchae sauce.
Blanch the Spinach in boiling lightly salted water for 30 seconds, plunge into ice water, squeeze out excess moisture and season with a pinch of Salt and a teaspoon of the japchae sauce.
Prep the vegetables: slice the Carrot into thin matchsticks, thinly slice the Onion, julienne the Red Bell Pepper, and cut the Scallions into 1–2 inch lengths.
Thinly slice the Beef Sirloin across the grain and toss with 1 tbsp of the japchae sauce; let it sit to absorb flavor for 10–15 minutes.
Heat 1–2 tbsp Neutral Oil in a large skillet or wok over medium-high heat and quickly stir-fry the carrot until just tender; remove and repeat, cooking the onion, red bell pepper and the sliced shiitake separately until each is softened. Season lightly with a pinch of Salt and set the vegetables aside.
Add another tablespoon of Neutral Oil to the pan and stir-fry the marinated beef until just cooked through; remove and set aside with the vegetables.
Beat the Eggs with a pinch of Salt, make a thin omelet in a lightly oiled pan, then roll and slice it into fine strips to use as a garnish.
Return the cooked noodles to the large skillet or a big mixing bowl, pour in the remaining japchae sauce and a splash of the reserved mushroom soaking liquid if needed. Add the sautéed vegetables, beef and blanched spinach, then toss gently over low heat until everything is well coated and heated through. Taste and adjust seasoning with a little more Soy Sauce or Sugar if desired.
Transfer to a serving platter, scatter the egg strips and a tablespoon of Sesame Seeds over the top, and finish with a light drizzle of Sesame Oil before serving.
