Avocado Vichyssoise Soup Preparation Melt 2 tablespoons butter in a large pot. Add 3 peeled and cubed potatoes and 3 trimmed and chopped leeks. Cook for about 3 minutes, stirring, until softened. Add 4 cups stock. Boil, cover, lower the heat and simmer until vegetables are tender, about 20 minutes. Purée, then let cool. Stir in coarsely chopped flesh of 1 or 2 avocados before serving. Garnish: Chopped cilantro.