Cold water makes good oysters, and in winter, they're at their plump, luscious best. West Coast varieties, such as Kumamotos and Olympias, have a creamy sweetness we can't resist.
In a small bowl, whisk together champagne vinegar, lemon juice, horseradish, shallots, and sugar.
Season to taste with salt and pepper. Serve as a topping for oysters.