In a deep bowl, mash the garlic to a paste with a pestle or the back of a large spoon. Stir in the tahina. Then, with a whisk of spoon, beat in 1/2 cup of the cold water, the lemon juice, and salt. Still beating, add up to 1/2 cup more of water, 1 tablespoon at a time, until the sauce has the consistency of thick mayonnaise and holds its shape almost solidly in a spoon. Taste for seasoning. Taratoor can be served as a sauce with baked fish or fried cauliflower, and is used as an ingredient in hummus and baba ghannooj.