Using a small sharp knife, cut each prawn lengthways down centre without cutting right through. Open out to lay flat.
Preheat a lightly oiled barbecue or chargrill on high. Combined five spice, chilli, salt and pepper in a medium bowl. Add prawns and toss to coat. Spray prawns with cooking oil spray and cook for 2-3 mins each side , until just cooked through or to your own preference.
To make dressing, mix together the soy sauce, lemon juice and sugar. Place lettuce on serving platter. Scatter over cucumber and coriander leaves. Top with prawns and serve with lemon soy dressing.