This beautiful, seasonal spring soup is a wonderful canvas for whatever you have on hand. I like to finish it off with a little feta to highlight that Mediterranean idea behind the soup. Beautiful, wonderful tahini and sumac flavors really come through.
For the soup base: Melt the butter in a soup pot over medium heat. When the butter is bubbling and frothy, add the shallot and the celery and cook for 2 minutes. Add the diced potato, thyme, bay leaf, and stock. Turn the heat to high, bring to a boil, and then reduce to a simmer for 10 minutes until potatoes are tender.
Finish the soup: While the soup base simmers, snap the woody ends off the asparagus, about 2 inches from the root end. Discard those ends and cut the remaining spears into 1-inch long pieces. Add the cut asparagus to the pot, cover, and cook for 4 more minutes. Add the watercress, tahini, cream, salt, and sumac. Remove (and discard) the bay leaf and thyme sprigs and remove from heat. Carefully pour the soup into a blender and puree until smooth. Ladle into soup bowls.
Garnish the soup: Garnish the soup with feta, potato chips, scallions, aleppo pepper, olive oil, and serve.