If using fresh garlic, in a small bowl mix the lemon juice with minced garlic. Allow the mixture to “steep” and marinate for about 10 minutes. Strain the lemon juice into a medium sized bowl and discard the garlic.
Whisk the tahini with the lemon juice, garlic powder if using, and salt. The mixture will seize and become clumpy.
Add the ice cold water one tablespoon at a time, whisking with each addition, until the mixture is smooth. The finished sauce should be thin enough to pour but thick enough to dip, comparable to the thickness of stirred sour cream or labneh.
