I’ve seen Gordon Ramsay do a couple quick soups like this on the telly, so I’m certainly not claiming originality here. There’s variations to be had (I’m trying them out as able), but the basic deal is:

Slice super-thin: 2-3 new potatoes, the wee sweetish ones
1-2 cloves elephant garlic
Coarsely chop: 1 BIG bunch watercress
Barely boil: 2+ cups veggie stock

In a nice large pan, sauté the taters and garlic in a tablespoon or so of olive oil and (perhaps) a wee pat of butter, over med-high heat, until the taters just barely begin to tenderize. Lightly salt&pepper. Then toss in the ‘cress and roll around until the greenery just barely begins to just start to think of wilting. Lightly salt&pepper. Now turn off the heat and pour the barely-boiling stock right on top of the works. Stirstirstir for 37 seconds, or until ‘cress stops leaping around and settles down a bit.
Now transfer to blender and/or food processor and puree.
Now put in bowls and eat.
Now cunningly screw with your dinner guests by allowing them to assume that there’s 1/2 a gallon of heavy cream in there somewhere, somehow.
Shhhh… i won’t let on if you don’t.