This is how I make Japchae as a part of weekend meal prep. This recipe is dairy-free, vegan, nut-free, and gluten-free (use tamari instead of soy sauce) friendly.
See below recipe for roasting carrots and bell peppers.
Make the sauce by mixing soy sauce, maple syrup, sesame oil, garlic, and ginger.
Place cut tempeh in a saucepan and cover with water. Bring to a boil, then reduce the heat and simmer for 10 minutes. Drain the water.
Place simmered tempeh and 1/4 cup of sauce in a clean plastic bag. Seal the bag and shake to evenly coat the sauce on the tempeh. Let it sit on the counter to marinate.
Place sliced mushrooms and 1/4 cup of sauce in a clean plastic bag. Seal the bag and shake to evenly coat the sauce on the mushrooms. Let it sit on the counter to marinate.
Bring a large pot of water to boil (to cook vermicelli.)
Meanwhile, heat a couple of tbsp of oil in a pan. Add chopped onion and cook until it turns brown. Add chopped spinach and cook for another minute. Remove from heat and set aside.
Cook vermicelli following package direction minus a couple minutes. Drain the water. Add remaining sauce to the pot with vermicelli, and cook for additional couple minutes while continuously stirring. Remove from heat and mix in spinach and onion.
Heat a couple of tbsp of oil in a pan and cook marinated mushrooms. Do the same with tempeh until it turns brown on the edge.
Toss your desired amount of carrots, bell peppers, mushrooms, tempeh, and noodles together and serve. Store remaining ingredients in separate containers for later.
You can add a tbsp of sesame oil and a tbsp of soy sauce to vermicelli cooking water. Noodles are less sticky this way.