Heat the oven to 220C/425F/gas mark 7. Bake the potatoes and the onion for about an hour or so, until soft all the way through. Cut the potatoes in half lengthways and scoop out the flesh into a bowl, keep the skins; peel the onion and chop, pop into a bowl and set aside. Add 25g of the butter, crème fraiche, perl las, the salt and pepper to the potato and mash to combine.
Put the potato skins on an oven tray, divide 10g of the remaining butter between them and sprinkle with sea salt. Bake for 5-10 minutes until they crisp up, then remove from the oven.
Bring a saucepan half-filled with salted water to a boil, cook the shredded leek for 5 minutes, add the spinach and cook for further 30 seconds. Drain, refresh under cold water and drain again. Squeeze out as much water as you can, then fold into the mash. Pile into the empty potato skins, then return them to the oven for 15 minutes more, until the top of the mash is crisp and browned.
Melt the remaining butter in a small frying pan on a high heat for one to two minutes, until it starts to brown, then fry the chopped walnuts for 30 seconds, stir in the chopped onion and sultanas. Cook for 1 minute, stir in the parsley. Spoon over the potatoes and serve hot.
