Discard the seeds, chop the flesh and liquidize it or press it through a sieve.
Cover and freeze for about 1 hour, until the pulp is mushy.
Whisk the egg whites, salt, cream of tartar, lemon juice and sugar over a pan of simmering water until it is stiff.
Leave to cool.
Fold the meringue mixture into the partly frozen watermelon.
Cover and freeze for 1 hour.
Turn the mixture into a chilled bowl, beat well to break down ice crystals, cover and freeze for a further 2 hours.
The meringue mixture holds air in the fruit, so there is no need to soften the sorbet in the refrigerator before serving.
It's one iced dessert you can forget about till the last minute.
Serves 6
There's nothing more refreshing than a slice of chilled watermelon, the bright sunset pink flesh outlined by the rim of dark green rind. Extend the season by turning some of the fruit into sorbet and, for the best of both worlds, serve a slice of fruit and scoop of the perfectly complementary ice.
