Makes 1 generous cup
Combine yogurt, buttermilk, mayonnaise, chives, parsley, lemon juice and zest, 1/2 teaspoon salt, and cayenne in a bowl. Using the side of a knife blade, mash garlic and a pinch of salt against a cutting board repeatedly to form a mostly smooth garlic paste. Add garlic paste to bowl with remaining ingredients and stir well to blend. (Flavors will intensify as the sauce sits.) Sauce may be made up to 2 days in advance.