While the pasta water is coming to a boil, in a saucier or wok, gently sauté the garlic cloves and the parsley stems in a generous amount of olive oil.
When the garlic is just beginning to turn color, raise the heat and add the clams and a splash of white wine. Cover and let the clams steam, shaking the pan from time to time.
After a minute or two, uncover the pan and check on the clams to see if they've opened. If they have, using a slotted spoon or skimmer, transfer the clams to a large bowl. Once the clams are cool enough to handle, shell most of the them, leaving some unshelled for garnish.
There should be ample clam juices in the pan. Strain them if you see any sediment. Otherwise, remove the garlic and parsley stems, and add the crema di pistacchio and give it a stir to dissolve. Then turn off the heat until the next step.