Three hours before serving: make the dressing. Put the garlic and coriander in a
food processor with 1 tsp salt and the sugar, then pulse
to a pulp. Add the chilli, ginger and citrus juices and pulse
until smooth. Cover and chill.
Two hours before serving: cut the watermelon into segments, remove the skin
and seed the flesh. Cut into lozenges about 3cm wide.