Melt the butter in a saucepan over medium heat, then add the onion and fry for 5-8 mins until soft.
Add the cubed potatoes and 300ml of water and bring to the boil.
Reduce the heat and simmer for 10-15 mins.
Meanwhile, put the haddock in another pan and cover with the milk.
Cook gently for 5 mins, or until just tender.
Remove the haddock from the milk and place on a plate to cool.
Keep the cooking milk aside.
Flake the haddock into chunks.
Put the reserved milk and flaked haddock in the pan with the potato mixture and cook for another 5 mins. Season and sprinkle over the parsley/chives to serve.