Heat a large pan of water. Once it boils, add salt, stir and add the potato. After a couple of minutes, add the pasta and set the timer according to packet cooking instructions.
While the pasta cooks, saute the leek and a pinch of salt in the oil and butter, until very soft.
Once the pasta is cooked and the potato is soft and collapsing, use a spider sieve to lift both into the leek pan and toss everything well. Add the cheese, freshly ground black pepper and another knob of butter and toss again.
Serve topped with the chopped hazelnuts and more cheese, if you want.
