Marinate the Beef: In a medium bowl, combine beef with soy sauce, sugar, mirin, sesame oil, black pepper, and minced garlic. Mix well and marinate while preparing other ingredients.
Soak the Dried Mushrooms (skip this if you are using fresh mushrooms): In a medium bowl, cover the dried Shiitake mushrooms with boiling water. Cover with plastic wrap to stop heat escaping. Let the mushrooms sit for at least 20 minutes until the caps are tender. Before use: Squeeze the excess liquid from the mushrooms, then remove and discard any tough stems.
Japchae sauce: In a medium bowl, stir together soy sauce, sugar, sesame oil, sesame seeds, black pepper, and garlic. Set aside.
Cook the Vegetables:
White Onion: In a large skillet, heat about 1 teaspoon of oil over medium heat until shimmering. Cook the white onion with a pinch of salt until just tender (for about 2 minutes). Be sure to keep stirring to prevent burning. Remove from heat and set aside.
Carrot: Add ½-1 teaspoon vegetable oil and cook the carrots with a pinch of salt until just tender. Be sure to keep stirring to prevent burning. Remove from heat and set aside.
Red Bell Pepper: Add ½-1 teaspoon vegetable oil and cook the red bell pepper with a pinch of salt until just tender. Be sure to keep stirring to prevent burning. Remove from heat and set aside.
Green onion: Add ½-1 teaspoon vegetable oil and cook the green onion with a pinch of salt until just tender. Be sure to keep stirring to prevent burning. Remove from heat and set aside.
Shiitake Mushroom: Add the shiitake mushrooms into the skillet with no oil needed. Stir and keep cooking until the mushroom is fully cooked or browned. Remove from heat and set aside.
Cook the Beef:
In a large skillet, heat about ½-1 tablespoon of oil over medium-high heat until shimmering. Add beef and cook, stirring, until beef is cooked through. Remove from heat and set aside.
Cook the Noodles and Spinach:
Spinach: In a large pot of salted boiling water, cook spinach until tender, about 30 seconds. Remove the spinach from the heat and rinse in cold water a couple of times. Squeeze out excess water from the spinach with hands. Set aside.
Noodles: In the same pot, using the same water, cook the noodles according to the product packaging or until tender but still chewy. Drain the cooked noodles and transfer to a large platter or bowl.
Cook the Eggs:
In a large skillet, heat about 1 teaspoon of oil; swirl to coat over medium-low heat. Cook the beaten eggs, swirl so the egg covers the surface into a thin layer. Cook until the egg is set. Remove from the heat and cut into small pieces or julienne.
Mix all the ingredients with sauce:
Stir in about ⅔ of the Japchae sauce into the noodles until well coated. Add cooked vegetables and beef into the bowl, then add the remaining sauce. Toss all the ingredients together until well coated. Adjust seasoning with salt/pepper, sesame oil, and sesame seeds if needed.
Notes
To sweeten your Japchae, feel free to add extra sugar by mixing about 1 teaspoon of soy sauce with ¼ to ½ tablespoon sugar.