Place the fish in a pan and add the milk, a pinch of sugar and bring to simmering point, then take off the heat and set aside.
Pop the butter in a pan, sweat the leeks for 2 minutes, add the potatoes and some water to nearly cover the vegetables. Simmer gently for 5-7 minutes. By this time the water should have evaporated and you can add the flour. Stir well and cook on for a minute or two ensuring nothing burns.
Drain the fish and gradually add the flavoured milk to the potatoes and leeks. Gently squash some of the potatoes down but leave some chunks. Add the mustard and season with salt and pepper.
Flake the fish into large chunks and add to the soup, make sure there are no bones and add the parsley. Taste again, add some cream and serve with your favourite bread.
Although technically a soup, it should be chunky and is great for lunch or supper.
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