Add the tahini, lemon juice, garlic, 1/2 teaspoon cumin, salt, and several cracks of black pepper. Start blending, and with the motor running, stream in the ice water, a tablespoon at a time, until the hummus is thick yet smooth and creamy. Taste for seasonings and add more salt, lemon juice, garlic, or cumin as needed. If the hummus is still too thick for your liking, add a tablespoon or two more of ice water.