If you are buying whole crabs, then buy large ones and boil for a couple of minutes with salt; pick out the meat carefully ensuring that there are no shell pieces.
Add the finely chopped cucumber and onion to the crab meat.
In a large bowl, mix all the ingredients required for the dressing. Taste and adjust the seasoning as required (the measurements should only be taken as a guideline as the taste can vary according to the variety/brand of the ingredients).
Add ¾ ths of the dressing to the crab meat just before serving; season with salt and mix lightly. Taste and add more dressing if necessary.
Garnish with coriander leaves and serve immediately as a salad.