Put the buttermilk, mayonnaise, lemon juice, Worcestershire sauce, garlic, salt, and pepper in the bowl of a food processor outfitted with the metal blade. Process until perfectly smooth -- about 5 seconds. Add the cheese, and pulse just until blended. The cheese should remain fairly chunky.
Transfer the sauce to a bowl. If the sauce is too thin for its intended purpose (dipping, for instance), stir in the optional sour cream. Then stir in half the parsley. Garnish with the remaining parsley.
When covered and refrigerated, the sauce will remain fresh and wonderful for at least 5 days. Enjoy as a dressing for salad greens, as a dipping sauce for crudité or crackers, or as a spread for a humble hamburger.