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Line a sieve with a piece of muslin and set over a bowl. Tip in the ricotta, gather up the ends of the muslin and gently tie together. Leave to drain for 4 hrs or preferably overnight.
Meanwhile, bring a pan of water to the boil. Blanch the nettle leaves quickly, then drain and chill under the cold tap. Thoroughly drain again, and squeeze out as much water from the leaves as you can, then very finely chop and chill until needed.
To make the gnudi, transfer the strained ricotta to a large bowl. Beat a little, then add all but a few tbsp of the Parmesan, the egg yolks, nettles, a good grating of nutmeg and plenty of seasoning. Give it a good stir to combine. Tip the semolina into a large baking tray (it will need to fit in your fridge later). Wet your hands, dip them in the semolina and, working quickly, scoop 1 heaped tsp of the ricotta mixture into your hands and gently roll into a ball. Place the ball on the semolina tray and roll around so that it is completely coated. Pick it up and roll between the palms of your hands to create a smooth ball, then put back in the semolina. Continue with the rest of the mixture – you should have about 28 balls in total. Leaving the balls in the semolina, make sure that they are well spaced, then cover loosely with cling film. Chill for 12-24 hrs – the longer the better – until a skin has formed on the gnudi.
To cook, bring a large pan of water to the boil. Meanwhile, spoon the pesto into a frying pan. Once the water is boiling, drop in batches of the gnudi and simmer for 2-3 mins or until they rise to the surface. Scoop out with a slotted spoon and transfer to a sieve. Repeat with the remaining gnudi. Heat the pesto in the pan with a few tbsp of the gnudi cooking water, to loosen it. Tip the cooked gnudi into the frying pan and gently turn the balls in the pesto. Divide between plates and scatter over the remaining Parmesan and a good grinding of black pepper before serving.