Combine all ingredients for beef in a medium mixing bowl. Let marinate while preparing other ingredients.
Heat a large skillet over medium-low heat. Add 1 tsp oil; swirl to coat. Add beaten eggs swirl skillet to egg covers surface in a thin layer. Cook until it’s just set, about 2 to 3 minutes, then flip over and cook for another minute. Slip omelet onto a plate and let it cool.
Heat the same skillet over medium-high heat. Add 1 tsp oil and sliced onion. Season with a pinch of salt, sauté 4 to 5 minutes. Remove onion from pan, keep warm. (Place on a plate or a bowl where you are going to mix whole entire japchae.)
Add 1 tsp oil to the pan and red pepper with a pinch of salt. Sauté 1 minute. Remove chili pepper from pan, keep warm. Add 1 tsp oil to the pan, carrots and a pinch of salt. Sauté 2 to 3 minutes. Remove carrot from pan, keep warm.
Wipe off excess oil from the pan and add mushrooms. Sauté 2 tp 3 minutes. Remove from pan, keep warm.
Add 1 tsp oil and add marinated beef to the pan; sauté for 2 to 5 minutes or until beef is cooked through and all moisture has evaporated. Remove from pan, keep warm.
Bring a large pot of water to a boil. Add generous amount of salt. Add the spinach, cook about 10 seconds, not much longer. Move spinach from the boiling water to ice water immediately. Squeeze out water from spinach gently. Set aside with other veggies.
In the same water, cook the Korean sweet potato noodles. Boil noodles according to package directions or 6 to 10 minutes.
Meanwhile the noodles are cooking, whisk all ingredients for sauce in a bowl, set aside. Slice cooled egg omelet into thin ribbons and set aside. Drain the cooked noodles and place in a large mixing bowl.
Cut the noodles as needed, if you prefer. Pour 2/3 of the sauce into the noodles and gently mix until noodles absolve all of the sauce. Add the noodles into the vegetables and beef, toss gently to combine everything with your hand or/and tongs.
Garnish Japchae with some additional sesame seeds as desired. Serve hot, warm or at room temperature. Enjoy!