Trim the fish, remove any skin and cut the raw fish into small neat pieces. Place in a bowl and pour over the strained lemon juice. Place the fish in the refrigerator and leave to marinate for 4–6 hours, turning the pieces from time to time.
Peel and chop the onion. Scald the tomatoes and peel away the skins. Halve, remove the seeds and chop the tomato flesh coarsely. Halve, deseed and chop the green pepper. Add the onion, tomato, green pepper and chopped parsley to the fish with the oil and vinegar, a seasoning of salt and pepper and a dash of Tabasco sauce.
Chill for several hours more, then check seasoning and flavour before serving.