Directions

• Marinate the beef:
  In a medium bowl, combine the beef, soy sauce, rice wine vinegar, sugar, and garlic. Set it aside to marinate while you prepare the noodles.
• Cook the noodles: 
  Set a large pot of water to a boil. Cook the noodles according to the package instructions.
  Drain and rinse under cool running water for a few seconds to make them just cool enough to handle. Do not cool them down all the way. If you have long noodles, use kitchen shears to cut them into 8 to 10-inch lengths.
• While the noodles cook, make the japchae sauce:
  In a large bowl (big enough to do lots of mixing as the rest of the ingredients are added), whisk together the soy sauce, sugar, and sesame oil.
• Toss the noodles with the sauce:
  Place your cooked noodles into the japchae sauce and use tongs or salad tossers to mix. When the noodles have cooled enough to touch, I like to use my hands to evenly distribute all the sauce. Taste and adjust for salt and pepper. Set the bowl aside.
• Make the egg garnish (jidan):
  Separate the egg whites and yolks into two small separate bowls, and whisk each.
  Heat a medium nonstick pan over medium heat and add 2 teaspoons vegetable oil. Pour the egg yolks in, tilting the pan back and forth to cover the surface in a thin layer of egg. Cook until the top is just set, but the bottom is not browned. Flip and cook about 10 seconds more. Remove from heat and let cool. Once cooled, slice the egg into thin strips about 1/8 inch wide and about 2 to 3 inches long, and place on the noodles.
  Cook and slice the egg whites the same way, and place them on the noodles.
• Cook the vegetables:
  Heat a large heavy-bottomed skillet or wok over high heat. Cook each vegetable separately with 1 teaspoon oil and a pinch of salt until they catch some color and just begin to soften, 2 to 3 minutes. Transfer them to the noodles. 
  Cook them in the following order: onions, carrots, shiitake mushrooms, red bell pepper, light portion of green onions.
  Cook the dark green portion of the green onion followed by the spinach separately with 1 teaspoon oil and a pinch of salt until barely soft, about 30 seconds or less so that they don’t get watery. Transfer them to the noodles.
• Cook the meat: 
  Give the skillet or wok a couple of minutes to fully heat again over high heat. Add 1 teaspoon oil and the marinated meat, leaving behind the marinade in the bowl. 
  Cook for 2 to 3 minutes, stirring constantly to allow excess moisture to evaporate. Add the cooked meat to the noodles.
• Finish the japchae and serve: 
  Toss the noodles with the vegetables, eggs, and beef to combine well. Garnish with sesame seeds and serve.
  Refrigerate leftover japchae, tightly covered, for 2 to 3 days.