This is a thick yogurt sauce that is teeming with herbs. You can serve it with cold roast chicken, as a dressing for potato salad, or as a dip for vegetables. I adapted this recipe from one given by Sam and Sam Clark in their book Casa Moro. You can substitute other herbs for the dill and lemon balm, such as chervil, lovage, parsley, or tarragon, in any combination—though if you use tarragon, use less, because it is potent.