Meanwhile, melt the butter in a medium (10- to 11-inch) heavy-bottomed pot or Dutch oven, such as Le Creuset, over medium heat. Add the leeks, fennel and carrots and sauté over medium-high heat for 10 minutes, stirring occasionally, until the leeks are tender but not browned. Stir in the garlic and tarragon and cook for one minute. Sprinkle on the flour and cook, stirring constantly, for 2 minutes. Add 1/2 cup of the sherry, the chicken stock, 4 teaspoons salt, 1 1/2 teaspoons pepper and the Parmesan rind. Bring to a boil, lower the heat and simmer, partially covered, for 20 minutes.