Step 1Break up the head of garlic with your hands, letting the unpeeled cloves fall into a blender. Add the lemon juice and 1/2 teaspoon salt. Blend on high for a few seconds until you have a coarse purée. Set the mixture aside for 10 minutes to give the garlic time to mellow.
Step 2Pour 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 salt.
Step 3Whisk the mixture together until smooth (or use a food processor), adding ice water, a few tablespoons at a time, to thin 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 (up to 1 1/2 cups), whisking energetically until you have a perfectly smooth, creamy, thick sauce.
Step 4Taste and add additional salt and cumin if you like. If you're not using the sauce immediately, whisk in a few tablespoons ice water to loosen it before refrigerating. This makes about 4 cups tehina sauce, more than is needed for the remainder of the recipe. The tehina sauce will keep up to a week refrigerated, or it can be frozen up to a month.
Step 1Place the chickpeas in a large bowl with 1 teaspoon baking soda and cover with water (the chickpeas will double in volume, so use more water than you think you need). Soak the chickpeas overnight at room temperature. The next day, drain the chickpeas and rinse under cold water.
Step 2Place the chickpeas in a large pot with the remaining 1 teaspoon baking soda and add cold water to cover by at least 4 inches. Bring the chickpeas to a boil over high heat, skimming any scum that rises to the surface. Reduce the heat, cover the pot, and simmer until the chickpeas are completely tender, about 1 hour (timing will vary depending on the age of the chickpeas). Then simmer a little longer (the secret to creamy hummus is overcooked chickpeas; don't worry if they are mushy and falling apart a little). Drain.
Step 3Combine the chickpeas, tehina sauce, salt and cumin in a food processor. Purée the hummus until it is smooth and uber-creamy, several minutes. Then purée it some more!
Step 4To serve, spread the hummus in a shallow bowl, dust with paprika, top with parsley and more tehina sauce if you'd like, and drizzle generously with oil.