Marinate the prawns in half the lime juice and olive oil for an hour.
In the meantime, combine mayonnaise, lime zest and the rest of the lime juice in a bowl and stir.
Chop the coriander being careful not to bruise it.
Cook the prawns lightly in a fry pan with some olive oil and the rest of the lime juice until they have just curled.
Spread both the top and bottom slices of bread with the lime/mayonnaise mixture, then add prawns and a few coriander leaves. Slice crusts off for presentation and serve on a platter.