It exists!
Watercress does, that is. It is a cruchy (when raw) green. You might be able to get it at more upscale grocery stores that have good specialty vegetable departments. If you can't find it, use some good green, such as spinach for your soup.
Just make a good chicken soup (instructions below if you don't have a favorite recipe), chop up your watercress/greens, and pop them in your soup just before you serve it so that they are wilted but not obliterated.
Chicken Broth: Put chicken carcass/pieces in a pot (I use a chicken carcass after I have baked the chicken and cut the meat off of it for another meal). Lacking that, use a couple of chicken legs. Add about 3 stalks celery, cut up, 2 - 3 carrots, peeled and sliced, 2 garlic cloves, minced, and 1 onion, sectioned and flaked. Add sufficient water to cover the chicken, bring to a boil, lower the heat, and simmer for about 2 hours. Remove the chicken bones, let cool, and remove any meat from the bones. Flake the meat into small pieces and return to the soup. Add salt, pepper and herbs (I like rosemary) to taste. If you chill the broth, the fat will rise to the surface and you can peel it off before you reheat it, thereby reducing the fat content.
You now have a good chicken stock/soup base. For your soup, you can add other vegetables as you see fit - in your case, watercress - homemade or commercial noodles, or barley. Be creative!