In a blender puree the beans in batches with the tarragon and the broth and strain the puree through a fine sieve into a bowl, pressing hard on the solids. In a large saucepan cook the leek, the garlic, and the red pepper flakes in the oil over moderately low heat, stirring occasionally, until the leek is softened and begins to turn golden. Add the puree, the half-and-half, the lemon juice, and salt to taste and simmer the soup, stirring, for 5 minutes. Makes about 5 1/2 cups, serving 6 as a first course. Gourmet November 1991