Wash the parsley and basil leaves, pat dry well and chop finely. Peel and finely chop the garlic. Chop the pistachios too. Grate the parmesan.
Put the ingredients in a blender (or other container) and process into a paste with the addition of olive oil with a hand blender. Add enough olive oil until the pesto has the desired consistency. Season to taste with salt and pepper.