Prepare the Noodlesn a large pot, bring water to a boil. Add the sweet potato noodles and cook for 6-8 minutes until the noodles are soft but slightly chewy. Drain and rinse the noodles under cold water to stop the cooking process.
Cut the noodles into more manageable lengths (about 6-8 inches) and set them aside in a bowl. Toss with 1 teaspoon of sesame oil to prevent sticking.
Cook the VegetablesHeat 1 tablespoon of vegetable oil in a large skillet or wok over medium heat. Add the minced garlic and stir-fry for about 30 seconds until fragrant.
Add the onion, carrots, red bell pepper, and shiitake mushrooms. Stir-fry for about 4-5 minutes until the vegetables are tender but still slightly crisp. Remove from the skillet and set aside.
Cook the SpinachIn the same skillet, add a little water and bring to a boil. Blanch the spinach for about 30 seconds until wilted.
Drain, squeeze out any excess water, and set aside.
Cook the BeefIn the same skillet, add the thinly sliced beef. Stir-fry for 3-4 minutes until fully cooked. Season with salt and pepper. Set aside. Mix Everything TogetherIn a large mixing bowl, combine the noodles, cooked vegetables, spinach, and beef. Add soy sauce, sugar, sesame oil, and sesame seeds. Toss everything together until the ingredients are evenly coated with the sauce.
Optional Egg GarnishBeat the egg in a bowl and cook it in a non-stick pan to make a thin omelet. Once cooked, slice the egg into thin strips and use it as a garnish for your japchae.
Serve and EnjoyTransfer the japchae to a serving platter and garnish with the egg strips (if using). Sprinkle additional sesame seeds on top for extra flavor and texture.