For the syrup
Mix the lime powder and water in a large bowl.
Peel the rind off the watermelon and trim any fruit so you’re left with only the white flesh. Soak the rinds in the water overnight (the lime keeps the flesh firm and crunchy).
Prepare the syrup by mixing the sugar, ginger and water in a large saucepan. Stir over a low heat until the sugar has dissolved then bring to the boil.
Drain the rinds, discard the water, and add the rinds to the syrup. Simmer until the rinds are just soft.
Decant into sterilized bottles, seal and allow to cool upside down.
Serve the preserve as part of a cheese board.