In a medium bowl, combine the seasonings for beef and whisk all together. Add the beef and toss to coat evenly. Set aside to marinade for at least 10 minutes.
Thinly slice the onion and shiitake mushrooms.
Cut the carrot and bell pepper into thin julienne strips.
Heat 1 tsp. sesame oil in a large frying pan over medium heat. Add beaten eggs to the pan and swirl the pan to cover the surface of the pan in a thin layer. Cook until it’s just set, about 2 minute. Then flip over and cook for another 2 minute. Transfer the omelette to a plate to let cool.
In the same pan (without washing), heat 1 tsp. sesame oil over medium heat. Add onion and a pinch of salt and sauté till tender. Remove the onion from the pan to a large plate.
In the same frying pan, repeat this process for shiitake mushrooms, carrot, and bell pepper.
In the same pan, add marinated beef and sauté until cooked through and all moisture has evaporated. Remove from pan to the large plate.
When the omelette is cooled, roll up and cut into thin ribbons and set aside.
Bring a large pot of water to a boil and add 1 tsp. salt. Add spinach, cook about 10-15 seconds (American spinach is more tender than Japanese spinach). Prepare iced water in a large bowl and move the spinach out of the pot to the iced water to stop cooking. Do not throw this boiling water. Squeeze out water from spinach as much as you can and set aside.
In a medium bowl, combine the seasonings for spinach and whisk well. Add the spinach and coat well with seasonings. Set aside. (Skipped for video tutorial)
In the same water that you blanched the spinach, add Korean sweet potato noodles. Boil noodles according to package directions or 6 to 10 minutes. Drain well and pour some sesame oil to coat noodles so they won’t stick. Place the noodles on the large plate/bowl and cut with kitchen shears as needed.
In a small bowl, combine all the ingredients for sauce and whisk well. Pour the sauce over the noodles.
Combine the sauce and noodles first so that noodles can absorb the sauce. Then toss stir-fried vegetables and beef with noodles together. Add half of reserved omelette and sesame seeds and toss together.
Transfer to a serving platter and garnish Japchae with the rest of omelette and more sesame seeds as desired. Serve hot or at room temperature. Enjoy!