Julienne your carrots and bell pepper, then chop your cabbage, mushrooms, green onions, and yellow onions. Set the vegetables aside for now, but do not combine them as we will be stir-frying them individually later.
Bring a large pot of water to a boil, then add in your dangmyeon noodles and cook for 12 minutes or until noodles are soft (these shouldn’t be al dente). When the noodles are done cooking, rinse them under cold water to cool them off and stop them from cooking further. When they’re cooled off, move them to a large mixing bowl.
Combine soy sauce, gochujang paste, toasted sesame seed oil, minced garlic, sugar, sesame seeds, and black pepper in a bowl and mix until consistent. Drizzle 6 tbsp of sauce over the noodles, then use your hands to evenly mix it in.
Using kitchen scissors, cut your noodles so that they are shorter and easier to grab/eat. Transfer them to a pan along with 1 tbsp of a neutral oil and stir-fry over medium-high heat for 4-6 minutes or until noodles start to become a little sticky.
Transfer noodles back to the bowl and then begin sauteeing each of your vegetables individually, adding more oil if necessary. Sautee the mushrooms last, drizzling 2 tablespoons of sauce onto them as they cook. This will help them absorb more of the flavor of the sauce.
After your veggies are all sauteed, add them to the bowl of noodles. Add in the remaining sauce and use your hands to mix all ingredients thoroughly.
And just like that your spicy vegan japchae is ready to eat! Serve while hot, or refrigerate and serve cold. These noodles work both ways!