Soak noodles in heated water for 12-15 minutes. Drain them and set them aside in a bowl.
Slice meat in 1/4″ thick matchsticks. Season using soy sauce, wine, and sugar. Set meat aside.
Heat 1 tbsp. of oil in a large-sized skillet on med-high. Add onions, carrots, peppers & 1 pinch of sea salt. Stir-fry till vegetables soften.
Add mushroom slices and another pinch of sea salt. Continue stir-frying till the mushrooms are soft, too. Add more cooking oil as needed.
Add the spinach. Stir-fry till it wilts. Remove the skillet from heat. Transfer vegetables to a large-sized plate.
Add 1 tbsp. oil to the same skillet and put on high heat. Add meat & stir-fry it until cooked fully. Transfer meat to a plate with vegetables and reserve juices in a skillet.
Add noodles to the pan with meat juice. Pour Japchae sauce atop the noodles. Combine by tossing till noodles are coated well. Allow noodles to cook on med. heat for 3 to 4 minutes, till they have softened, and the liquid of the sauce has been almost fully absorbed into the noodles.
Reduce heat level down to low. Add meat and vegetables back into skillet over noodles. Add the sesame oil and seeds. Toss everything together.
serve