Stir the saffron with the water in a small bowl. Gently whisk the mayonnaise, onion, lemon juice
and mustard together in a separate, non-metallic bowl until thoroughly combined. Season to taste
with salt and pepper and stir in the saffron soaking liquid. Cover with cling film and chill.
Pull the heads off the prawns and peel. Cut along the length of the back of each prawn and
remove and discard the dark vein. Rinse and pat dry with kitchen paper.
Arrange the lettuce leaves on a large serving platter or on individual serving plates. Top with
the prawns and scatter with the tomato wedges and olives. Serve with the saffron dressing.