Poaching the oysters a day ahead gives you a triple boon: There is no mess before serving, the poaching liquid makes a delicious base for your sauce, and the shellfish remain as fresh tasting as if they were just opened.
4 dozen large oysters on the half-shell, 1 cup liquor reserved
2 sticks (1/2 pound) plus 2 tablespoons unsalted butter
2 medium shallots, minced
1/4 cup dry white wine
3 tablespoons Champagne vinegar or white wine vinegar
1/2 cup chopped fresh ginger
Dash of cayenne pepper
Unless you are very handy with an oyster knife, ask the fish market to open the oysters for you. Have them pack them on the half-shell on ice and reserve the oyster liquor separately. (Preopened oysters that come in containers are often mushy and unpleasant.) Just before you cook the oysters, take them out of their shells. Rinse them briefly under cold running water only if there is grit or pieces of shell on them. Reserve the shells.
In a large skillet, melt 2 tablespoons of the butter. Add the shallots and cook over moderate heat until softened and translucent but not browned, 1 to 2 minutes. Add the wine, vinegar, and reserved oyster liquor. Bring just to a simmer. Add the oysters and poach, turning once, until the edges just begin to curl, about 1 minute. If in doubt, remove the oysters earlier rather than later (they will be cooked again). Put them in a covered container and refrigerate at once.
Add the ginger to the poaching liquid in the skillet and boil until the liquid is reduced to 3 tablespoons. Strain into a small bowl, pressing on the ginger to extract as much flavor as possible. Cover and refrigerate.
Boil the oyster shells in a large pot of water for at least 15 minutes; drain. When they are cool enough to handle, rinse them well under running water, rubbing off any pieces of muscle that cling to the shell. Drain well; then refrigerate in a plastic bag. (The recipe can be prepared to this point up to a day ahead.)
Before cooking the oysters, arrange the shells in a single layer in several roasting pans. I use disposable ridged aluminum foil broiling pans; each pan will hold 16 oysters.
Shortly before you plan to serve the oysters, remove them from the refrigerator; drain, reserving the liquid; and set 1 in each shell. Pour any accumulated liquid into a small heavy saucepan. Add the 3 tablespoons reserved poaching liquid, bring to a boil, and cook until reduced to 2 tablespoons. Whisk in the cayenne and 2 tablespoons of the butter. Remove from the heat and beat in 2 more tablespoons butter until just blended. Return to low heat as necessary and continue whisking in the butter, 2 to 3 tablespoons at a time, making each addition after the previous one is almost incorporated, until all the butter is added and the sauce is thick and emulsified. Set over a pan of warm water (it will hold for about 20 minutes).
Heat the broiler. Spoon a little dab of the beurre blanc over each oyster, to coat. In batches as necessary, broil the oysters just until they are heated through, 1 to 2 minutes; do not overcook. Spoon a heaping teaspoon of the remaining sauce over each oyster and serve at once.