Mix the grated zest and the juice of the limes and oranges in a large bowl. Stir in a pinch of salt and sugar. Peel the onions, cut into wafer-thin rings and add to the juice. Clean and slit the chili peppers and remove the seeds. Wash and finely chop. Add to the juice or season it as hot as possible with Tabasco. Score, blanch and peel the beefsteak tomato. Halve, core, dice the halves and add to the marinade and stir thoroughly. Bring the salt water to the boil. Wash and add the prawns, leave to stand for 4 minutes in a closed saucepan over low heat. Drain, rinse in cold water and remove from the shells except for the tail. Carefully remove the bowel. Let the prawns stand in the marinade. Halve very thick prawns lengthways.