Make the Melba toast. Toast the bread on both sides, then cut off the crusts. Cut through the thickness of each slice of toast from one side to the other to give you 2 thin slices. Cut these into triangles and place under a preheated grill until they curl up. Watch carefully because they can burn very quickly. If you are not using the Melba toast the same day, store it in an airtight container once cool. It will keep well for a few days
Peel the carrot and onion and chop into large pieces. Trim the celery and chop into large pieces. Pick the leaves from the tarragon and chop them. Peel and finely chop the shallots.
Put the crab in the freezer for an hour or so before cooking.
The devilled crab can be kept in the fridge for up to 3 days, but for the best flavour my advice is to buy the crab and cook it and serve it on the same day. If you do refrigerate it, be sure to remove it an hour or so before serving.