In the bowl of a food processor, combine the tahini, garlic, lemon juice and salt and process on low speed for 2 minutes or until thoroughly incorporated. Turn the speed to high and blend until the tahini mixture begins to whiten. Gradually add up to 1/2 cup water until the mixture reaches the desired consistency. Transfer the sauce to a serving bowl and garnish with the parsley. Leftover tahini sauce can be stored, tightly covered in the refrigerator, for up to 2 weeks.