Cook the sweet potato starch noodles according to the package instructions. Drain and rinse under cold water. Cut the noodles into shorter lengths with kitchen scissors and set aside.
In a small bowl, mix together the soy sauce, sugar, sesame oil, and minced garlic until the sugar is dissolved.
In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat.
Add the beef strips and stir-fry until cooked through. Remove from the skillet and set aside.
In the same skillet, add the remaining vegetable oil.
Add the sliced onion, julienned carrot, and bell peppers. Stir-fry for 2-3 minutes until they begin to soften.
Add the shiitake mushrooms and stir-fry for another 2-3 minutes.
Add the blanched spinach and chopped green onions. Stir to combine.
Return the cooked beef to the skillet with the vegetables.
Add the cooked noodles and pour the sauce over everything.
Toss all the ingredients together until well combined and heated through.
Drizzle the sesame oil over the top and sprinkle with sesame seeds.
Add the sliced egg strips and gently toss to combine.
Transfer the japchae to a serving dish.
Enjoy your meal!