Bring two large pots of salted water to a boil. One will be for the pasta and the other for the nettles.
When the water comes to a boil in one of the pots, "carefully" add the nettles and give them a stir. Cook for 5 minutes and transfer them to a colander with a slotted spoon. You want to leave any dirt or grit in the cooking water. Let the nettle drain.
Add the pasta to the other pot of clean water and cook until al dente.
While the pasta is cooking, press most of the water out of the nettles, transfer them to a food processor and puree.
Drizzle in the olive oil and process until completely smooth.
Add the ricotta, lemon zest and juice and herbs. Pulse the processor to blend all the ingredients.
Season with salt and pepper.
Remove a cup of the water the pasta is cooking in and reserve. Drain the pasta and then return it to the pot.
Toss in the nettle ricotta cheese mixture and stir to combine.
Add the fresh sorrel and a little of the reserved pasta water to create the desired consistency of the sauce.
Stir in the walnuts and serve.