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, mix together lime juice, jalapeño, sugar, salt, and lime zest.
Pour mixture into a metal baking pan and freeze.
Use a fork to scrape the frozen mixture into an icy mound and spoon it onto oysters.