1. Preheat the oven to 410°F. Put the potatoes on a baking tray and bake for 1–1½ hours, until the skins are crispy and the insides soft. Remove from the oven and allow to cool until you can handle them. Reduce the temperature to 390°F.
2. Heat the butter in a frying pan over a medium. Once the butter melts add the leek and fry until very soft – about 20 minutes – stirring occasionally. Put the leek into a bowl with the remaining filling ingredients and stir to mix well.
3. Halve the potatoes and scoop most of the flesh into a bowl – leave about 5mm of flesh against the skin. Fill each potato skin with the fish filling and place on a baking tray – or use a 12-hole bun tin so the filled potato halves don’t fall over.
4. Mix the potato with the topping ingredients until fairly smooth then blob it on top of the fish filling in each potato skin. Bake for 20-30 minutes, until the potato is slightly coloured.