From Zahav: A World of Israeli Cooking, ©2015 by Michael Solomonov and Steven Cook. Reproduced by permission of Houghton Mifflin Harcourt. All rights reserved.
Break up the head of the garlic with your hands, letting the unpeeled cloves fall into a blender. Add the lemon juice and ½ teaspoon of the salt. Blend on high for a few seconds until you have a course puree. Let the mixture stand for 10 minutes to let the garlic mellow.
Pour the mixture through a fine-mesh strainer set over a large mixing bowl, pressing on the solids to extract as much liquid as possible. Discard the solids. Add the tehina to the strained lemon juice in the bowl, along with the cumin and 1 teaspoon of the salt.
Whisk the mixture together until smooth (or use a food processor), adding ice water, a few tablespoons at a time, to thing it out. The sauce will lighten in color as you whisk. When the tehina seizes up or tightens, keep adding ice water, bit by bit (about 1 ½ teaspoons more salt and cumin if you like. If you’re not using the sauce immediately, whisk in a few tablespoons of ice water to loosen it before refrigerating. The tehina sauce will keep a week refrigerated, or it can be frozen for up to a month.