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Heat oven to 220C/200C fan/gas 7. Mash the garlic with 50g butter. Slice the bread first one way, then the other (in a hedgehog pattern), but do not cut all the way through. Stuff the crevices with garlic butter, then wrap loosely in foil, leaving the top open to crisp, and bake for 20 mins while you make the chowder.
Heat a knob of butter in a saucepan with the oil. Tip in the celery and onion, cook for 5 mins or until soft, then add the flour and mix to a paste. Pour in the milk, a little at a time, and cook until smooth and the consistency of double cream. Add the potatoes and half the mackerel, cover and simmer for 15 mins until the potatoes are cooked.
Flake the remaining mackerel, add to the chowder with the sweetcorn and simmer for 1-2 mins until the sweetcorn is cooked. Splash in a little water if it is too thick. Ladle into bowls, scatter over the celery leaves and serve with the bread for dunking.