Place the stock, lemongrass, ginger and soy sauce in a medium deep-sided frying pan over medium heat and bring to a simmer. Simmer for 5 minutes. Remove the lemongrass and ginger and discard. Add the noodles and cook for 4 minutes. Remove from the broth and divide between bowls. Add salmon and asparagus to the broth and cook for 1 minute. Top the noodles with the salmon and asparagus and spoon over the broth. Serve with pickled ginger and coriander leaves. Serves 4.