Step 1: Prepare the Sweet Potato Noodles

1. Bring a large pot of water to a boil and add the sweet potato starch noodles.
2. Cook according to the package instructions, usually about 6–8 minutes, until the noodles are soft and chewy.
3. Drain the noodles and rinse them under cold running water to stop the cooking process.
4. Toss the noodles with 1 teaspoon of sesame oil to prevent them from sticking together and set them aside.

Step 2: Prepare the Vegetables and Protein

1. Blanch the spinach in boiling water for about 30 seconds, then drain and squeeze out excess water. Set aside.
2. Heat 1 tablespoon of vegetable oil in a large pan over medium-high heat.
3. Add the onions and sauté for 1–2 minutes until slightly softened.
4. Add the carrots, bell peppers, and mushrooms, stirring frequently until the vegetables are tender but still vibrant.
5. If using cooked beef, chicken, or tofu, add it at this stage and stir-fry for another 1–2 minutes.

Step 3: Make the Sauce

1. In a small bowl, whisk together soy sauce, sesame oil, sugar, minced garlic, black pepper, and rice vinegar (if using).
2. Taste the sauce and adjust seasoning as needed.

Step 4: Combine Everything

1. Add the cooked noodles to the pan with the vegetables and protein.
2. Pour the sauce over the noodles and toss everything together until evenly coated.
3. Add the blanched spinach and chopped green onions, mixing gently to incorporate.

Step 5: Garnish and Serve

1. Transfer the Japchae to a serving platter.
2. Sprinkle toasted sesame seeds over the top.
3. If desired, garnish with thinly sliced cooked egg yolk for a traditional presentation.
4. Serve warm or at room temperature. Enjoy!