Set a large pan of well-salted water (10g salt per litre) on to boil. If you diced accurately, the potato will take 8-10 minutes to cook until slightly mushy, completely tender, but not quite disintegrating. If you’re unsure, test a few cubes in the water before proceeding.
Depending on the cooking time of the pasta, add the pasta and potatoes in order to the water, so they will both be cooked at the same time (the pasta al dente, the potato completely soft). If you are feeling less gung-ho, you could cook the potato first, drain it, and keep it warm while the pasta cooks.
Put the oil, pepper and your chosen herb in a metal bowl and, if the bowl fits, warm them together over the water as the pasta cooks. Don’t let the water boil over.
Drain the pasta and potatoes, reserving some of the cooking water, and add them to the bowl of oil. Toss vigorously, adding splashes of pasta water so some of the potato starts to break down and form an emulsified sauce.
Serve immediately, with a little cheese on top if you like.
