In a large pot, melt the butter and saute the onions until they begin to color. Stir in the flour to form a roux and lower the heat. Cook a few minutes, stirring constantly. Add the broth, stirring all the time, until the soup base begins to thicken. Add the watercress and simmer about 10 minutes. Puree the soup in a blender and return it to the pot. In a medium bowl, beat together the cream and eggs; beat in 2 cups of the hot soup. Stir this mixture back into the soup pot. Season with lemon juice, salt and pepper. Heat thoroughly, but do not boil. Garnish each boil with a watercress sprig if desired.