Find the reddest watermelon you can for this granita.
1 kilogram watermelon flesh, pips removed
3 tablespoons white rum or vodka or to taste
150 grams granulated sugar
150 ml water
2 tablespoons chopped fresh ginger
1 stalk lemongrass, thinly sliced
zest of 1 lemon
Cut the watermelon into small pieces and place on a freezer-proof tray. Cover and freeze until solid.
Put the sugar, water, ginger, lemongrass and lemon zest in a saucepan and bring to the boil, stirring to dissolve the sugar. Cook for 2 minutes, cool and refrigerate to chill. Strain through a sieve to remove the solids before using.
To serve: Put half the chilled syrup, half the frozen watermelon and half the alcohol in a blender or food processor and blend until thick and smooth. Place in the freezer while you process the remaining
ingredients. Pour into chilled glasses and serve immediately.