Start by making the granita. Do this at least three hours in advance. Mix the raspberry syrup with the prosecco or cava, the raspberry vinegar, and the lime zest in a bowl. Stir well and place the bowl in the freezer uncovered.

Let the mixture freeze for about 45 minutes. Remove the bowl from the freezer and stir with a fork to loosen the frozen edges. Return the bowl to the freezer and repeat this process about every 45 minutes. After around three hours, and a few stirs, the granita will be fully crystallized.

Store the granita in an airtight container in the freezer. It will keep for about two weeks. Before serving, you can fluff it again with a fork to loosen the ice crystals.

Opening the oysters requires some care. Find the point where the two shells meet and carefully insert the oyster knife. Gently pry the shell open. If needed, detach the oyster from the shell so it rests nicely in its own liquid.

Serve each oyster with a teaspoon of granita and garnish with some cress.