Watercress is a great all-purpose green that can be eaten raw or cooked, and I used to buy it regularly until I found a cheap source of arugula which can be used similarly. Normally if I found a bag of watercress I would want to use it for sandwiches and salads, but since I also have several boxes of sprouts and prepared salads, it made more sense to cook it down into soup.
I started by chopping some garlic and onion and sauteing them in olive oil until brown, then added a diced yellow potato and peeled and diced granny smith apple. For a more classic soup, omit the garlic and apple and use a larger potato. I used both because I had only one small potato. I added about a cup and a half of chicken bone broth I made a few months ago from rotisserie chicken carcasses and an equal amount of water, then added a bag of watercress, enough to fill the pot to the top.
I seasoned this with salt*, pepper, and thyme* (a lot of salt and pepper, because my broth was unsalted and sweet) and boiled it until the potatoes were soft, then blended the soup with an immersion blender and let it boil for a few more minutes.
I was originally going to serve the soup with lemon, but it came out sweeter than expected, so I used shaved Parmesan and black pepper instead.