Peppery, bright green watercress adds a touch of spring to a classic leek and potato soup. If you can make this the night before it’s needed, so much the better, as the flavors improve from standing overnight.
Heat the oil in a large soup pot. Add the leeks and sauté over medium heat, covered, until they just begin to turn golden. Stir occasionally.
Add the potatoes, bay leaf, bouillon cubes, and just enough water to cover. Bring to a simmer, then simmer gently, covered, until the potatoes are tender, about 15 to 20 minutes.
Mash some of the potatoes against the side of the pot with the side of a spoon. Add the watercress and rice milk and simmer over very low heat for 10 minutes longer. Season with salt and pepper.
Allow to stand off the heat for an hour or two before serving, or let cool and refrigerate overnight. Heat through before serving. Stir in the parsley. If needed, adjust the consistency with more rice milk, then adjust the seasonings.
Serve, garnishing each bowlful with a generous amount of watercress.