Making it well, however, requires a certain level of skill.
Watercress is an aquatic weed that has an aggressive peppery flavor. It's been commercially cultivated forever so it's not hard to find.
The problem with greens is that overcooking turns them from a bright green color to a drab olive-green. The reason has to do with the science of chlorophyll which is tied up in cells. When the cellular structures are broken typically in an acidic environment, the magnesium in the chlorophyll gets displaced by hydrogen ions which changes the color.
The choices to preserve the color are pretty much all poor.
You could quickly blanch the vegetables and dunk them in an ice-bath which preserves the cells. You could cook the vegetables in a slightly alkaline environment but that has a tendency to turn them into mush and taste soapy. You could cook them for the barest amount of time while watching the clock and reheat them to the barest minimum.
You can easily see how the word "skill" would attach itself to these three choices.
2 bunches watercress
1 tbsp butter
1 large onion
1/4 cup peas (frozen is fine)
3 cups water (read the notes below)
Note 1: The CC prefers that the taste of the watercress be dominant. Broth is too aggressive. A dashi made with just kombu is ideal but water is preferred over most broths.
Note 2: The watercress leaves must be plucked. This is a tremendous pain in the ass because the plant is basically a weed and grows copiously in all directions so you will have to work hard to strip the leaves.
Note 3: If the CC may use the oxymoron that is "conventionally unorthodox", the peas would be it. They help with the nutrition and the color.
Heat the butter in a vessel. Toss in the chopped onions and sautée for a while. Add the potatoes and let them cook for a bit. Add the broth and let it come to a boil. Add the peas and let cook until the potatoes and peas are tender.
This is the critical step.
Bring the mixture to a boil. Toss in the watercress and turn off the heat.
Purée with an immersion blender or with a regular blender if you don't have one. You can pass the mixture through a sieve if you want a refined presentation or just leave as is.
If you need to reheat, make sure you do at the barest of temperatures just up to the serving point.
Garnish with chives. Serve at once.