Melt butter in heavy large saucepan over medium heat. Add leek and saute until tender, about 5 minutes. Add stock and potato and bring to boil. Reduce heat and simmer for 25 minutes, stirring occasionally. Puree soup in batches in processor or blender. Return soup to the saucepan. Add half-and-half and bring to simmer. (Can be prepared 1 day ahead, covered and refrigerated. Bring to simmer before continuing.) Gradually add all cheeses to soup and whisk until melted. Ladle soup into bowls and garnish with tarragon.