Melt the butter in a large dutch oven over medium heat. Once melted, add the leeks, celery, and dried thyme. Season with salt and black pepper. Stir. Cook for about 5 minutes or until the veggies are tender. Add the garlic and potatoes, cook for 2 minutes before stirring in the vegetable broth. Bring the soup to a boil and then simmer for 20 minutes.
After 2 minutes, stir in the garbanzo beans and tahini. Simmer for 5 more minutes until the beans are warm and the tahini is fully incorporated.
Taking an immersion blender (or transfer to a regular blender), blend the soup until smooth and creamy. Season to taste with salt and black pepper, and then add the heavy cream. Cook for 5 more minutes until nice and warmed through.
Keep the soup warm on the stove or serve immediately topped with suggested toppings. Enjoy!