Pour the Guinness, the juice of the lemon, and the lemon zest into a shallow plastic container that can be placed in the freezer.

Place the mixture in the freezer for 1 hour or until it is frozen.

Shuck the oysters before taking the Guinness mixture out of the freezer.

Take the plastic container out of the freezer and scrape it with a fork until the ice turns into granita.

Spoon a small amount of the granita into each oyster and eat quickly before it melts. Season with a bit of black pepper to taste.