Finely chop the pistachios in a food processor or coffee grinder. Set aside one quarter of the ground nuts for garnish.
In a large skillet, over medium heat, sauté the onion in the olive oil and butter until it’s golden. Add the prosciutto and cook one minute; it should not become crispy. Add the pistachios and toss until well-coated with the other ingredients. Add the white wine and cook for 3 minutes. Add the cream, salt and pepper. Cook until the cream is slightly reduced, 4 or 5 minutes.
Bring a large pot of salted water to a boil. Cook the pasta until al dente (about 2 minutes less than package directions), drain, and retain 1 cup of the cooking water. Add the pasta to the skillet with the pistachio mixture and stir together for about a minute. Add some of the retained cooking water if it seems too dry.
Serve, garnished with the remaining chopped pistachios.
If you cannot find unsalted pistachios at the market, rinse the shelled pistachios in cold water to remove the salt and loose skins and pat dry