Melt butter in a saucepan over medium heat, then add onion and fry gently until transparent. Cook for about 5 minutes but do not allow to brown.
Add potatoes and water and bring to boil. Simmer for 10-15 minutes.
Meanwhile in another pan, cover the haddock with the milk and cook gently for about five minutes until just tender. Remove from the milk and, when cool enough to touch, flake gently into large pieces, removing any bones.
Add milk and flaked fish to saucepan containing potatoes and other ingredients and cook for a further 5 minutes. Season with salt and pepper and sprinkle with chopped parsley.
Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.