Put the coriander seeds and galangal or ginger in a saucepan. Pour in the stock, bring to the boil, then simmer gently for 5 mins. Leave to stand for 10 mins.
Meanwhile, cook the noodles following pack instructions. Drain and keep warm. Return stock to the boil. Add the fish sauce, chillies and garlic, reduce the heat and simmer for 2 mins. Add the prawns and salmon, return to a simmer and cook gently for about 5 mins until both are firm and cooked. Add the spring onions, herbs and lime juice, to taste.
Divide the noodles between soup bowls. Using a slotted spoon, lift out the prawns and fish, and place on the noodles. Season the hot stock, pour into the bowls and serve with Spring rolls on the side.