Bring 300ml water to the boil in a small saucepan. Add sugar and stir until dissolved. Remove from heat and set aside to cool completely.
Once cooled, add lemon juice and pepper, then stir to combine. Pour into a shallow tray and freeze (6 hours or overnight).
Scrape frozen surface with a fork to make granita, then top oysters with 1 tsp granita and serve.
