Put the haddock in a pan and add the milk- it should just cover the fish so use less or more. Bring to the boil, turn down to a simmer and cook for 5 minutes then set aside.
Melt the butter in a separate pan then add the onions and sweat over a low heat until soft and translucent. Add the potatoes and water, bring to the boil and simmer until the potatoes are almost cooked.
Remove the fish from the milk. Pour the milk through a sieve into the pan with the potatoes and cook for five minutes, Meanwhile flake the fish, discarding the skin and bones.
Crush the potatoes gently with a fork or potato mashes. Add the flaked fish and the cream and simmer for 5 minutes.
Pour into warm bowls and scatter with the chives to serve.