Mushroom Marinate:

1. Remove the stems and thinly slice the shiitake mushrooms. In a bowl, combine mushroom slices, 1 tbsp soy sauce, 1 tbsp sugar, 1 tbsp sesame oil, and 1 tsp minced garlic.
2. Mix well to ensure the mushrooms are evenly coated. Set aside and allow them to marinate while you prepare the other ingredients.

Boil Noodles and Spinach:

1. Boil the sweet potato noodles according to the package instructions. (I add them to a pot with boiling water for 2 minutes). Rinse the noodles under cold water and add 1 tbsp sesame oil to prevent them from sticking. Cut the noodles 3 or 4 times.
2. In a separate pot, blanch the spinach in boiling water for a minute.
3. Drain the spinach, then rinse them under cold water to stop the cooking process.
4. Set aside.

Stir Fry Vegetables:

1. Heat 1 tbsp of oil in a large pan or wok over medium-high heat.
2. Add sliced onion and stir fry until softened. Cut red bell pepper and carrot into matchsticks.
3. Add the carrots and mushrooms and stir fry for 1 minute.
4. Incorporate spinach, noodles, and sliced scallions.
5. Add the marinated mushrooms to the pan with the stir-fried vegetables.
6. Toss in the blanched spinach and cooked noodles.
7. Mix all the ingredients together, ensuring they are well combined.

Prepare the Sauce:

1. In a small bowl, whisk together 3 tbsp soy sauce, 1 tbsp sugar, black pepper and 2 tsp sesame oil until the sugar is dissolved. 
2. Pour the sauce over the vegetable and noodle mixture in the pan.
3. Gently stir and toss everything together, ensuring the sauce coats all the ingredients evenly.
4. Sprinkle sesame seeds over the Vegan Japchae for added texture and flavor.