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.
Mix the melon and prawns and add the spring onions,
lemongrass, ginger, lime zest and lime leaves. Chill.
To serve: toss the dressing and herbs into the salad and scatter
with chilli and peanuts.