Gather all the ingredients.

In a medium bowl, combine the seasonings for the beef: 1 Tbsp Korean soy sauce, 1 Tbsp sugar, ½ Tbsp toasted sesame oil, ½ Tbsp mirin, and 1 clove garlic (chopped). Whisk it all together. Cut ½ lb beef into thin strips 3 inches (7.6 cm) long. Add the beef to the bowl with the sauce and toss to coat evenly. Set aside to marinate for at least 10 minutes.

Thinly slice ½ onion and 5 shiitake mushrooms.

Cut ½ carrot and ¼ red bell pepper into thin julienne strips.

Now, make the omelette. (If doubling the recipe, make 2 omelettes with 2 eggs each.) Heat a large frying pan on medium heat. When the pan is hot, add 1–2 tsp toasted sesame oil and distribute it evenly in the pan (use less for a nonstick pan, more for a regular pan). Next, beat and add 2 large eggs to the pan and swirl them around to cover the pan‘s surface in a thin layer. Cook until it’s just set, about 2 minutes. Then flip the egg over and cook for another 2 minutes. Transfer the omelette to a plate to let cool.

Next, we‘ll cook the vegetables. Make sure to cook each different vegetable separately, starting with the light-colored ones and ending with the dark-colored ones, so that they will retain their beautiful colors. In the same pan that you just used (without washing), heat 1–3 tsp toasted sesame oil over medium heat. Add the sliced onions and a pinch of salt and sauté until tender. Remove the onions from the pan to a platter large enough to eventually hold all the cooked ingredients.

In the same pan, repeat this process with the shiitake mushrooms next, followed by the carrot strips, and ending with the bell pepper strips. As you finish cooking each ingredient, place in a separate pile on the platter.

Now, cook the beef. Using the same pan over medium heat, heat 1–3 tsp toasted sesame oil and add the marinated beef and sauté until cooked through and all the moisture has evaporated. Transfer from the pan to the large platter with the cooked vegetables.

When the omelette is cooled, roll it up and cut it into thin ribbons. Set aside on a separate plate.

Bring a large pot of water to a boil. In the meantime, prepare a large bowl of iced water. Once the water is boiling, add 1 tsp Diamond Crystal kosher salt and 12 oz fresh spinach. Cook for about 10–15 seconds (American spinach is more tender than Japanese spinach). Use tongs or chopsticks to transfer the spinach from the pot to the iced water to stop the cooking. (You‘ll cook the noodles next in this boiling water, so do not discard it.) Gather the spinach and squeeze out as much water as you can. Add the spinach to the platter with the other cooked ingredients.

Add 14 oz Korean sweet potato glass noodles (dangmyeon) to the same boiling water that you used to blanch the spinach. Boil the noodles according to the package directions, about 6 to 10 minutes. Drain well, transfer to a bowl, and toss with some toasted sesame oil to coat so the noodles don‘t stick to each other. Cut the long noodles with kitchen shears so it‘s easier to eat and they‘ll mix easily with the sauce and toppings. Then, add the noodles to the platter in a pile next to the rest of the cooked ingredients.

In a small bowl, combine all the ingredients for the sauce: ⅓ cup Korean soy sauce, ⅓ cup sugar, 2 Tbsp toasted sesame oil, 1 Tbsp toasted white sesame seeds, 1 clove garlic (chopped), and freshly ground black pepper. Whisk well. Pour the sauce only on top of the noodles.

Combine the sauce and noodles first so that the noodles can absorb the sauce (I used my gloved hand). Once the noodles are well coated, add in the stir-fried vegetables and beef and mix everything together.

Finally, add half of the omelette ribbons and some sesame seeds and toss together. Transfer to a serving platter and garnish the Japchae with the rest of the omelette ribbons and more toasted white sesame seeds, as desired. Serve warm or at room temperature. Enjoy!

You can keep the leftovers in an airtight container for up to 2 days. When you are ready to serve, reheat it in a frying pan with 1–3 tsp of oil and 1–2 Tbsp of water until the noodles become translucent again.