Boil water (without any salt) in a large pan. Separately prepare a bowl with very cold water and lots of ice and throw a handful of coarse sea salt in (a handful of salt for a kilo of prawns). Throw the shrimps or langoustines into the boiling water. Wait for a minute or two until they turn pink (the precise timing depends on the size of the prawn). As soon as they are pink take them off the boiling water and throw them into the salty icy water. Wait for a couple of minutes and serve them ( or keep them in the fridge covered with a damp tea cloth). They come out perfect.
According to my foodie brother then reason this works is that you create a process of osmosis with the salt and ice. My physics teacher mother agrees. If my mother says so it must be true!
