Preparation Preheat the oven to 350°F. Pour the pistachios onto a sheet pan and toast in the oven for 5 to 10 minutes. Remove and transfer 4 tablespoons of pistachios to a blender or food processor. Add the oil, vinegar, 1/2 cup water, parsley, salt, and pepper to taste, and blend the vinaigrette until smooth. Finely chop the remaining 1 tablespoon pistachios and add to the vinaigrette. Keeps well if refrigerated for up to 3 to 4 days.