Cut tips off the asparagus. Cut the remaining asparagus into 1-inch pieces, discarding any woody pieces from the bottom. Bring a 4-quart pot of water to a boil. Reduce to a simmer and add the asparagus pieces and tops. Blanch for 1 minute, then remove with a slotted spoon and place in a bowl of ice water. Using the same water, blanch the watercress for 30 seconds. Strain and place in bowl of ice water. Discard blanching water. Reserve several of the asparagus tips for soup garnish.

Place 4 cups of stock in a pot. Add asparagus, watercress and potatos. The vegetables should be covered with stock. If they are not, add additional stock as needed to cover.

Simmer for approximately 25 minutes or until the potatos are cooked through. Puree soup in batches in a food processor or a high-speed blender. During the pureeing process, add the yogurt or sour cream to the soup. Season with nutmeg, salt and white pepper. Divide into soup bowls and garnish with the reserved asparagus tips. If desired, garnish with thinned Greek yogurt and a blanched or grilled fresh ramp.