Cut the onion into slices. Add to a big pot of salted water and bring to the boil. Add the prawns and cook for 2-3 minutes. Not much longer than that!!!! Drain and cool in the fridge.
Serve cold from the fridge with wedges of lemon and mayonnaise for dipping.
Eat like this: Drizzle lemon over the prawns, Remove head and tail. Peel off the skin from the remaining body. Eat as it is or dip into mayonnaise.