Heat the oil in a large pot over medium heat. Add the onion, celery, and ginger and sauté until the onion is tender and translucent, about 8 minutes.
Stir in the broth, carrots, potato, and curry powder and bring to a boil. Season everything with salt. Decrease the heat to maintain a simmer. Cook, stirring occasionally, until the carrots are tender, about 25 minutes.
Using an immersion blender (or using a regular blender and working in batches), blend until completely smooth. Return the soup to the pot over low heat and cook, stirring often, until heated through.
In a small bowl, stir together the raw tahini, water, and salt to taste. It will seem chunky and weird at first, but keep at it until you have the consistency of coffee cream/half 'n' half. Add more water if necessary. Pour half of the tahini cream into the pot of soup and stir to incorporate.
With the remaining tahini cream, scrape half of it into a small bowl. Add the turmeric to the small bowl and stir to combine.
Serve the soup hot and garnish with swirls of the regular and turmeric tahini cream. Drag a paring knife through the swirls for a marble effect, if you like.