Melt the butter in a saucepan over medium heat, then add the onion and gently fry until clear and translucent.
Add the potato cubes and 300ml fresh cold water and bring to the boil. Reduce the heat slightly and let simmer for 10-15 mins.
Step 2 : Prepare the haddock
In another pan, add the smoked haddock and cover with the milk. Cook gently for 5 mins, or until haddock is just tender.
Remove the haddock from the milk with a slotted spoon (keeping the milk), transfer to a plate and leave to cool slightly.
When cool enough to touch, flake into large pieces, removing any bones.
Step 3 : Final touches
Put the saved milk and haddock flakes in the pan with the potato mixture and cook for another 5 mins.
Season with salt and chives to serve.