Poach Finnan haddie in the milk until lightly cooked. Flake onto a plate and reserve, discarding skin and bones.
Heat oil and butter in pan to soften leeks.
Add potatoes and water. Bring to boil.
Reduce heat and simmer, lid on, 10-15 minutes until potatoes are soft.
Add infused milk from the fish along with the flaked haddock and heat through.
For a smooth soup, lift out some of the mix with a slotted spoon and set aside for garnish. Blitz the remainder in pan and adjust thickness if wished by adding a little more milk. Taste and season – usually no added salt required.
Ladle into heated bowls, adding the spoonful of fish mix and finish with cream and herbs.