Boil water to cook the noodles. Cook the garlic and onion in oil. Cook the onion for 2 minutes and add the cabbage, shiitake mushrooms, and half of the scallions. Stir in the gochugaru and seafood and cook for 1 minute. Add the stock, soy sauce, and salt and bring the soup to a boil. Simmer until the seafood is cooked, about 6 to 7 minutes, and turn the heat off. Divide the jjamppong soup and seafood among the two bowls then add in to the cooked noodles and top with the remaining scallions. Enjoy!