Creating Japchae is a simple process if you follow these steps:

1. Prepare the Noodles: Boil a pot of water and cook the sweet potato starch noodles according to package instructions. Drain and rinse under cold water. Set aside.
2. Marinate the Protein: In a bowl, combine sliced beef or tofu with 1 tablespoon of soy sauce and a pinch of pepper. Let it marinate for at least 10 minutes.
3. Heat the Oil: In a large skillet or wok, heat 1 tablespoon of vegetable oil over medium-high heat.
4. Cook the Protein: Add the marinated beef or tofu to the skillet and stir-fry until cooked through, about 3-4 minutes. Remove from skillet and set aside.
5. Sauté the Vegetables: In the same skillet, add the remaining tablespoon of oil. Sauté the onions, carrots, and bell pepper for about 5 minutes until softened. Add the mushrooms and garlic, and cook for an additional 2 minutes.
6. Add Spinach: Toss in the spinach and cook until wilted, about 1-2 minutes.
7. Combine Ingredients: Return the cooked protein to the skillet, along with the noodles.
8. Season: In a small bowl, mix the remaining soy sauce, sesame oil, and sugar. Pour this mixture over the noodle and vegetable mixture, stirring well to combine.
9. Cook Together: Stir-fry everything together for another 3-4 minutes, allowing the flavors to meld. Adjust seasoning with salt and pepper to taste.
10. Serve: Once the noodles are heated through and the dish is well-combined, remove from heat. Sprinkle sesame seeds and chopped green onions on top for garnish.

These steps will guide you in creating this incredible dish effortlessly.