Rosti

Boil the potatoes in their skins until tender and leave to cool overnight (or at least until the potatoes are thoroughly cool before continuing). Peel the potatoes then grate them lengthwise (in order to get the longest strips) using a Rösti grater or a grater with large holes. Put into a mixing bowl.
In a heavy frying pan (such as a cast-iron one), heat 15 g (½ oz) of the butter with 1 tablespoon of the oil. When the butter foams, add the onion and cook gently until soft. Add the onions to the potatoes along with the salt and with a fork gently mix it all together, being careful not to squash the potatoes.
Heat 25 g (1 oz) of the butter and the remaining oil in the pan. When the butter foams, add the potato mixture and spread it out. Turn the heat to low, and after a few minutes gently turn the mixture with a spatula (it doesn’t need to form a single cake yet, so you can turn it in any way). Cook for a further few minutes, then turn once more. Now gently press down on the mixture to form a cake. Leave the rösti to cook on low heat for about 10 to 15 minutes.
By now there should be a light golden crust forming around the very edge of the rosti. Break the remaining 30–40 g (1–1½ oz) of butter into little pieces and distribute it around the rim of the pan. Turn the heat up a bit higher and continue cooking for a further 5 minutes.
Slide the spatula around the rim of the rösti and shake the pan gently to loosen the bottom. Now turn the rösti upside down onto a serving dish; the top should have a beautiful golden-brown crust.