Step 1In a small food processor, mince the garlic. Add the parsley and chop it by pulsing a few times. Hold the cilantro sprigs together and cut it in 2 or 3 pieces to fit in the food processor; add the cilantro to the processor and chop it. Add one-fourth cup of water. Blend the mixture to a puree (if you have trouble pureeing the garlic and herb mixture, add an additional tablespoon or so of water for extra liquid, or add a spoonful of the tahini paste so there is enough substance to puree).
Step 2Spoon the tahini paste into a medium bowl. Add 2 tablespoons lemon juice, the garlic herb puree and salt to taste. Stir to blend well. Stir in 1 tablespoon lemon juice and 1 tablespoon water. The sauce should be thinner than mayonnaise; if it is too thick to pour, gradually stir in more lemon juice or water, depending on how tart you would like the sauce. Taste, and add more salt or lemon juice if desired. Add cayenne pepper to taste, or serve the sauce with cayenne sprinkled on top.
Step 3The sauce thickens when refrigerated. Just before serving, if necessary, stir in more water by teaspoons to bring the sauce back to the consistency you like.