Spinach and Oyster Bisque

Preparation

1. Drain the oysters in a fine sieve set over a measuring cup, add enough water to the oyster liquor to measure 2 1/2 cups, and reserve the liquid. Discard any shell fragments from the oysters, rinse the oysters briefly, and chop them into 1/2-inch pieces. In a kettle cook the onion in the butter over moderate heat, stirring occasionally, until it is pale golden, add the flour, and cook the roux, stirring, for 2 minutes. Whisk in the reserved oyster liquid and bring the mixture to a boil, whisking. Add the spinach, half the oysters, and the tarragon and simmer the mixture, stirring, for 2 minutes. In a blender purée the mixture in batches until it is smooth, transferring it as it is puréed to a bowl, and return it to the kettle. Add the cream and the remaining oysters and heat the bisque over moderate heat, stirring, until it is hot and the oysters are cooked, but do not let it boil. Stir in the zest, the sugar, the nutmeg, and salt and pepper to taste and serve the bisque with the oyster crackers.