Put the beets into a steamer basket over a large pot of boiling water. Cover and steam for about 45 minutes or until beets are tender. Drain and let cool until you can handle them. Cut the ends off of the beets and peel them (the peel should rub off easily with a little help from a paring knife here and there). Cut the beets into large dice.
Toast the walnuts in a dry saute pan over medium-high heat for 3 minutes or until fragrant, and set aside.
Put the watercress, goat cheese, buttermilk, vinegar and salt into a food processor and process until smooth and creamy.
Put the beets on a serving dish, drizzle the watercress sauce on top and sprinkle with walnuts.