Cut the cucumber into four equal chunks, then cut 3 of the chunks in half lengthways (keep the fourth for another day) to give you 6 halves. Trim off the skin plus 3-4mm of flesh from the rounded parts of the halves so they will sit firmly, skin side down, on a plate. Brush with olive oil and season well.
Heat a griddle pan to very hot and grill the pieces on both sides until dark char marks appear: these add extra flavour.
Scoop out and discard the flesh of the preserved lemon and finely chop or shred the skin. Add the lemon juice and some olive oil to make a dressing and season. Add the shrimp and coriander and toss to coat the shrimp. Arrange the cucumber on a plate, add a dollop of yoghurt and spoon the shrimp mix over.