Heat some oil in a wok or heavy-based frying pan, add the garlic, shallots, lemon grass and ginger and stir-fry over a medium heat for 2 minutes until just turning colour.
Add the stock and coconut milk to the pan and bring to the boil. Add the sweetcorn and cook for 5 minutes.
Stir in the crab meat, fish sauce, lime juice and sugar and cook to heat thro