In a large, heavy bottom saucepan over medium heat add the butter and leeks and cook until the leeks are tender, 5 to 7 minutes. Once the leeks are tender add the flour, stirring well and cook until all of the flour has been incorporated. Add the stock, cauliflower and the potato and bring the mixture to a boil. Once boiling, reduce the heat to medium-low and simmer for 15 to 20 minutes. Remove from the heat, and let cool for about 5 minutes. Add the soup, in batches, to a food processor or blender and puree. Return the soup to the original saucepan over medium-high heat, add the cream and bring to a simmer. Reduce the heat to low and simmer for 10 to 12 minutes to allow the soup to thicken. Stir in the sage. Ladle the soup into serving bowls and garnish with tarragon or chives.