For gin and tonic granita, blend cucumber, gin and 10ml sugar syrup in a blender until smooth. Pass through a fine sieve, stir in tonic water and transfer to a 20cm x 30cm shallow metal tray and freeze, stirring and scraping occasionally with a fork, until ice crystals form (4-5 hours).
Combine cucumber and tonic in a bowl and refrigerate for flavours to develop (2-3 hours), then strain and discard liquid.
For gin cure, process all ingredients except sugar in a food processor until fine (2-3 minutes), then stir in sugar and salt to taste. Place kingfish in a non-reactive container and pat the cure mix evenly on both sides. Refrigerate for 1-2 hours for flavours to cure.
For juniper mascarpone, mix ingredients in a bowl, season to taste and refrigerate until needed.
Wipe excess cure from kingfish with paper towels and pat dry. Cut kingfish into 5mm slices, arrange on a platter, top with granita, diced cucumber and salad burnet and serve with juniper mascarpone.
