03Place 1 oyster on each of 20 oyster shells or place 2 oysters in each of 10 small ramekins. Top oyster in each shell with 2 tablespoons leek mixture, or top oysters in ramekins with 1/4 cup leek mixture. Place on rimmed baking sheet. (Can be made 4 hours ahead. Cover and chill.)
04Preheat oven to 500°F. Divide breadcrumbs among shells or ramekins. Bake until leek mixture bubbles and crumbs are golden, about 8 minutes.