Trim the leeks and chop, keeping the white and green parts separate. Melt the butter in a large saucepan and gently fry the white parts and half the green parts for 5 minutes. Add the potatoes, then stir in the cider, stock and mustard and bring almost to the boil. Reduce the heat and cook gently for 20 minutes until the potatoes are soft but still holding their shape.
Flake the smoked mackerel into small pieces, discarding any skin and stray bones. Add to the pan with the chopped chives, nutmeg and remaining green leeks. Simmer gently for 5 minutes.
Stir in half the crème fraîche and season to taste with salt and pepper. Spoon into bowls, top with the remaining crème fraîche and garnish with chive sprigs.