Peel ginger and grate the ginger
Lightly bruise the lemongrass with the handle of a large knife to release its flavour and chop into smaller pieces.
Place the ginger and lemongrass in a small pan with the water and maple syrup and over a low heat slowly bring the liquid to the boil.
Simmer for 8 minutes.
Remove from heat, strain and leave to cool completely.
7. Divide custard apple between 4 glasses. Add the lime juice and zest to the cooled syrup.
Pour equal amounts of syrup over custard apple and serve.