Add a drop of the oil and whisk until completely combined.

Gradually add the oil, a drop at a time, then in a thin trickle, whisking continuously until you have a thick mayonnaise.

Whisk in the lemon zest and juice, stir in the crabmeat and season to taste.

This mix can be made a few hours ahead and kept in the fridge.

2)For the Melba toast, heat the grill to high.

Place the bread on a baking sheet and toast on both sides until lightly browned.

Cut the crusts off the toast, then carefully cut through the middle of each slice horizontally to give you 2 thin slices.

Cut each of these into 4 triangles. Put the triangles, untoasted-side up, on the baking sheet and place back under the grill to brown – if they curl up slightly, all the better.

The toast can be made a day ahead and kept in an airtight container.