1) Bring 5 litres of water, 3 tablespoons of salt and the lemon to the boil in a large saucepan. Add half the prawns and reduce the heat to medium. Cook uncovered for only 3 minutes or until the prawns are barely cooked through. Remove with a slotted spoon to a bowl of cold water.
2) Bring the water back to a boil and repeat with the remaining prawns. Allow to cool, then peel and devein the prawns.
3) In a separate bowl, whisk together the mayonnaise, mustard, wine or vinegar, 1 tsp salt, pepper and dill. Combine with the peeled prawn. Add the red onion and celery and check the seasonings.
4) Serve immediately, or cover and refrigerate for a few hours.