1. Cut the potatoes into even-sized pieces, steam them until tender and then drain them.
2. Mash as finely as you can – a ricer would be even better for a fluffier gnocchi.
3. Place the potato in a bowl and add the egg, then sieve in the flour little by little.
4. Add the nutmeg, olive oil and salt, then bring everything together into a dough.
5. Turn out on to a floured surface and knead a few times to break down the gluten in the flour.
6. If it’s a little bit wet, you can add a little more flour to firm it up.
7. Divide the dough into four balls, then roll into lengths the size of a thick sausage. Cut into 3 cm lengths, then press down gently to flatten just a little so they cook evenly. Put in the fridge on a floured tray until you need them .
8. To make the sauce, melt the butter and then add the flour. Cook over a gentle heat for three minutes, then add the stock, little by little, whisking all the time to ensure there are no lumps.
9. Add the milk, then whisk in the black pepper, half the Parmesan and a little salt. Cook for a couple of minutes, then set aside.
10. Heat some of the olive oil in a frying pan; you will have to cook these in batches.
11. Fry the gnocchi for about two minutes on each side, slipping a few sage leaves into the pan to flavour the oil. Keep one batch warm while you do the other.
12. Serve the gnocchi on a pool of the sauce, garnished with the sage leaves and with the remaining Parmesan scattered on top.