Method
Put the butter into a large saucepan and allow it to melt over a low heat. Add the chopped onion, celery and leek to the pan and cook gently for about 10 minutes, until softened. Put the potatoes into the pan and stir to mix, then pour in 400ml of the milk and add the stock. Bring the pan up to the boil, then reduce the heat and simmer for 15 minutes, or until the potatoes are tender. While the vegetable are simmering, put the fish in a separate saucepan, pour over the remaining milk and bring up to the boil. As soon as the fish has cooked (it’s ready when it becomes opaque and flakes easily - this won’t take long), lift it out of the pan with a slotted spoon and set it aside, then pour the milk into the pan containing the veg. Tip the sweetcorn into the veg pan, too, and heat through for a minute or two. Next, take the pan off the heat, ladle half of the mixture into a food processor and blend until smooth. Return the blended mixture to the saucepan and place over a low heat. Carefully flake the fish with a fork, then fold it into the Chowder. Continue to heat the Chowder until it is properly hot all the way through, then garnish with parsley and serve.