Smooth and flavorful and rich and creamy... there is a lot going on in this simple spring and summer soup. It highlights and yet gently tames the peppery taste of watercress, those dark flavorful spring greens.
- If the watercress came, as it so oftens does, in a clump with the roots still attached, cut off and discard the roots. Keep as much of the stems attached to the leaves as possible—they are full of peppery cress flavor! Rinse the watercress clean, if needed: Put the watercress in a large bowl, rinse with cool water, lift the watercress out of the water (leaving any dirt and grit behind at the bottom of the bowl), and dry the watercress in a salad spinner or on several layers of paper towels.
- Trim the green onions and finely chop them.
- In a medium pot over medium heat, heat the oil until it shimmers. Add the green onion and sprinkle with the salt. Cook, stirring frequently, until the onion is soft, about 3 minutes.
- Meanwhile, trim and chop the celery. Add it to the green onions and cook, stirring frequently, until softened, another 3 minutes.
- Pour in the broth and bring to a simmer. Add the watercress and bring to simmer again. Remove from the heat and let sit about 5 minutes.
- Purée the soup using an immersion blender or in a blender or food processor, working in batches as necessary (hold a kitchen towel over the blender or food processor to contain any splatters and prevent possible burns). For the smoothest soup possible, run the purée through a fine-mesh sieve or a food mill.
- Whisk in the yogurt. Add salt to taste, if needed. Add freshly ground black pepper to taste. Cover and chill for at least 1 hour and up to overnight.
Serve chilled, garnished with fresh herbs, if you like, and a drizzle of olive oil.
* The need for plain, unsweetened yogurt should be clear (sweet vanilla isn't quite what you want in this soup), but the creamy and extra flavorful and smooth nature of whole-milk yogurt really puts this soup over the top as compared to using low-fat or nonfat yogurt. Know that to get a thick, yogurt-like texture, versions that have had most of their fat removed often have thickeners added, which can mess up their ability to merge well with other foods.