Wash and cut the rhubarb stalks into 1 cm / 0.4 inches pieces. Poach the rhubarb by adding it to a saucepan with about 500 ml / 2 cups of water, sugar and ginger. Cover and cook on a low heat for approximately 15 minutes until the rhubarb has softened. Blend in a mixer then strain through a fine-mesh sieve. Pour the mixture into a high baking dish and put in a freezer for about 4 hours.
Using a fork, stir the mixture every 40 minutes, breaking up any ice chunks of the mixture, until your rhubarb granita is frozen, this will take about 4 hours.
Scoop into chilled serving glasses and top with lemon zest and thin slices of fresh strawberries. For an extra kick, you can add a spoon of elderflower cordial or a spoon of white rum.