Place a large, heavy saucepan over medium heat and add the olive oil. Add the ramps and the chopped watercress stems to the olive oil and stir to mix. Sauté until wilted, which may take about ten minutes. Add the chicken stock and the potatoes to the pot and bring the liquid to a boil. Reduce the heat and simmer until the potatoes are tender. This should take about a half hour. Remove from heat and allow to cool slightly.
Mix the sour cream and the heavy cream together thoroughly and set aside.
Working in batches to avoid overfilling and splattering, ladle the soup into a blender or processor. To each of the batches, add some of the watercress leaves and some of the cream mixture. Puree each batch and then strain through a sieve into a large mixing bowl. Use a rubber spatula to press the puree through the sieve. Taste the puree and add salt and pepper to taste. Add the lemon juice.
Allow to cool to room temperature, then place in the refrigerator.
Before serving, taste the soup for seasoning and consistency. If too thick, thin with a small amount of cold milk or chicken stock. It should be thick and creamy. Ladle into chilled bowls and garnish with a few reserved watercress leaves.