Wash the nettles thoroughly. Transfer to a large saucepan with the water that is clinging to them, then leave to wilt over a medium heat for about 5 minutes, until tender. Leave to cool a little, then squeeze as dry as you can. Weigh out 250–300g squeezed nettles, chop them finely, then put them in a bowl. Add the melted butter and mix together well.
In a large bowl, lightly beat the ricotta with a fork, then sift in the flour. Add the egg yolks, nutmeg, parmesan and the cooled, buttered nettles and mix thoroughly (but do not over-mix: it’s nice to have a textured mixture rather than an homogenised paste). Taste for seasoning, adding salt and pepper only if required. Chill the mixture for at least an hour, until stiff.
Lightly dust a baking tray with plain flour. Using 2 dessertspoons, mould the mixture into gnocchi: take a small amount in one spoon and then scrape it off with the other spoon. Repeat this a few times, passing the mixture between the 2 spoons. Once you have the knack you will be able to make little rugby ball shapes. The gnocchi should all be the same size, about 2cm in diameter. Place on the floured baking tray. (At this stage they can be covered in clingfilm and left in the fridge for up to 24 hours if necessary.)
Bring a large pan of water to the boil, then lower the heat to a simmer. Gently place the gnocchi in the water in batches of 6 or 7 – it is important not to overcrowd the pan – and cook gently for 5–6 minutes. By this time they should all have floated back up to the surface. Remove carefully with a slotted spoon and place on kitchen paper to drain off excess water.