Make the pasta dough - blitz all the ingredients in a food processor, and if necessary add a little water to bring together into a dough. Knead the dough until glossy and smooth. Wrap in cling film and chill.
Prepare the filling. Wearing very durable gloves, put the nettles into a saucepan and boil for 2-3 minutes. Drain and set aside a few leaves for garnish, then chop the rest very finely. Combine in a bowl with the rest of the filling ingredients, season with salt and pepper and set aside.
Take the pasta dough and divide into 8. Roll out each section as thinly as possible using a pasta machine or by hand. Taking one long section of dough, place a teaspoon of filling at 2-inch intervals. Brush around the filling with water then place a second section of dough on top of the first and press down firmly around the blobs of filling. Cut either into squares or use a round pastry cutter to make individual ravioli. Repeat with all the remaining filling and dough - I made 18 ravioli out of these quantities which is enough for 6 as a starter. (You can also freeze the ravioli at this stage).
To make the sauce, combine the créme fraiche, mustard, lemon juice, butter and capers in a small saucepan and heat gently until just bubbling.
Bring a large pan of salted water to the boil and drop in the ravioli. Boil for 5-7 minutes until the pasta is tender. Drain and drizzle with the sauce, then scatter over the remaining nettle leaves and parmesan-style cheese shavings and serve immediately.