Bring a large Pot of water to a boil. Cook the 8 oz dangmyeon (fideos de papa dulce coreanos) noodles according to package directions, usually 6–8 minutes, until tender but still slightly chewy. Drain in a Colander, rinse briefly under cold water, and drain well. Use Kitchen Shears to snip the noodles a few times so they're roughly 6–8 inches long — this makes them much easier to toss and eat.

In a small Mixing Bowl, whisk together the ¼ cup salsa de soya, 2 tbsp azúcar, 2 tbsp aceite de sésamo, 1 tbsp semillas de sésamo tostadas, and ¼ tsp pimienta negra molida fresca until the 2 tbsp azúcar dissolves. This is your master sauce. Pour about half of it over the drained noodles in a large Mixing Bowl and toss well to coat. Set the remaining sauce aside.

In a Mixing Bowl, combine the thinly sliced beef with the ¼ cup salsa de soya, 2 tbsp azúcar, 2 tbsp aceite de sésamo, and 2 dientes de ajo picados minced. Mix well and let it marinate for at least 10 minutes while you prep the vegetables.

Heat a large Skillet or Wok over high heat. Add a drizzle of ½ tsp aceite neutro para cocinar el huevo and cook the marinated beef in a single layer for 2–3 minutes, stirring once or twice, until cooked through and lightly caramelized. Transfer to the large bowl with the noodles.

Return the Skillet to medium-high heat. Add 1 tbsp of ½ tsp aceite neutro para cocinar el huevo. Add the 1 cebolla amarilla mediana cortada finamente and 2 zanahorias medianas julienadas with a small pinch of ½ tsp sal dividida, para sazonar las verduras and stir-fry for 2–3 minutes until just softened and lightly golden. Transfer to the noodle bowl.

Add the remaining ½ tsp aceite neutro para cocinar el huevo to the Skillet. Add the 4 oz champiñones shiitake frescos, sin tallos, sombreros cortados finamente and stir-fry for 2–3 minutes until tender and fragrant. Add the 1 pimiento rojo mediano cortado finamente en tiras and 3 tallos cebollas verdes cortadas en trozos de 2 pulgadas and cook for 1 more minute. Season with a small pinch of ½ tsp sal dividida, para sazonar las verduras. Transfer to the noodle bowl.

In the same hot Skillet with no additional ½ tsp aceite neutro para cocinar el huevo, add the 4 oz espinaca fresca tierna enjuagada and a pinch of ½ tsp sal dividida, para sazonar las verduras. Toss quickly for 30–60 seconds just until wilted. Remove from heat and gently squeeze out any excess moisture with your hands or a clean towel. Add to the noodle bowl.

Separate the 2 huevos yolks and whites. Season each lightly with a pinch of ½ tsp sal dividida, para sazonar las verduras and beat them separately. Heat a small Skillet over low heat with a tiny amount of ½ tsp aceite neutro para cocinar el huevo. Cook the egg whites in a thin layer like a crepe, then cook the yolks the same way. Let both cool slightly, then slice into thin strips (jidan). Set aside for garnish.

Pour the remaining sauce over the large bowl of noodles, beef, and vegetables. Using Tongs or clean hands, toss everything together thoroughly until well combined and evenly coated. Taste and adjust with a little more ¼ cup salsa de soya or 2 tbsp aceite de sésamo if needed.

Transfer the japchae to a Serving Platters (Large) or individual bowls. Garnish with the egg white and yolk strips and an extra sprinkle of toasted 1 tbsp semillas de sésamo tostadas. Serve warm or at room temperature — japchae is delicious both ways.