cook the white and light green parts of 2 sliced leeks in 2 T unsalted butter in a medium pot over medium heat, stirring until tender, 4-6 mins.
add one chopped russet potato, 2 cups chicken broth, and 4 cups water.
simmer until potato is tender, 8 – 10 mins. add 2 bunches watercress (thick stems removed) and 1 T fresh tarragon leaves.
cook until watercress is bright green, about 1 minute.
puree, stir in 1/2 c buttermilk, season with salt. serve warm or room temperature topped with tarragon.
from a clipping in a magazine (not sure which magazine!)
it’s more of a watercress vichyssoise, or cold potato and leek soup, than a buttermilk soup. it doesn’t taste like buttermilk – it has a lot of flavor for being so simple. my mom hates tarragon, so i substituted dill. i figured, you can’t go wrong with potatoes and dill. i also added garlic. it came out well. if you are craving a cold potato soup, this is a good one. not my favorite soup, but definitely a good, solid way to use an abundance of cress. i liked sipping it cold out of a mug on a hot morning.