Put the fruit, orange juice and zest, liqueur and water in a food-processor and whizz until smooth.
I like to leave the seeds in but you can sieve your mixture if you prefer.
If you have an ice-cream maker, churn the mixture until it starts to freeze.
Alternatively pour the mixture into a rigid container and freeze, stirring every 30 minutes or so until ice crystals start to form and it feels slushy.
Whisk the egg whites until stiff peaks form, then gradually add the sugar, beating until firm and glossy.
Fold the meringue mixture into the fruit mixture and continue to freeze and stir until completely frozen.
If made in advance, transfer the mixture to the fridge 30 minutes before serving.