Begin by soaking sweet potato glass noodles in a large bowl of hot water for about 30 minutes, ensuring they soften and expand. Stir occasionally to prevent sticking. Once they are fully pliable, drain the noodles and set them aside.

Heat a tablespoon of oil in a large pan or wok over medium-high heat. Add thinly sliced beef and stir-fry for about 3 to 4 minutes, or until it’s browned and cooked through.

In the same pan, add another splash of oil if needed, then toss in your colorful vegetables such as julienned carrots, bell peppers, and spinach. Stir-fry for about 3 to 5 minutes until they're tender yet vibrant.

Reintroduce the cooked beef to the pan along with the soaked and drained noodles. Pour in soy sauce and sesame oil, stirring vigorously for an additional 3 to 5 minutes.

Once everything is well mixed and heated, transfer the Korean Japchae to a serving dish, sprinkling chopped green onions on top.