Heat the oil in a large pot over medium. Add the onion, celery, and thyme; cook, stirring occasionally, until translucent but not browned, about 5 minutes. Add the cauliflower, potato, broth and 1 1/2 cups water; increase the heat to medium-high and bring to a boil. Immediately reduce the heat to medium-low, cover and simmer until the cauliflower and potato are fork tender, about 25 minutes. Uncover the pot; increase the heat to medium, stir in the tahini, and simmer 10 minutes longer. Remove from the heat. Puree in batches in the blender or using an immersion blender. (Soup may be made in advance up to this point and reheated).