TO MAKE THE PESTO: In the bowl of a food processor, process the walnuts and garlic until finely ground. Scrape into a serving bowl and add the ricotta cheese, parmesan cheese, olive oil, parsley and salt. Mix well and set aside at room temperature.

TO MAKE THE PASTA: Cook the pasta according to package directions until al dente. Before draining, reserve about 1/4 cup of pasta water. Add the pasta to the pesto and mix well. Mix the pasta water into the pasta if the pesto seems too thick, adding it a tablespoon at a time.

Gently toss in the butter, peas and prosciutto. Season to taste with salt and pepper and serve right away.