Cut the tomatoes over a bowl to catch the liquid. Remove the seeds and save the juice. Pass the juice through a sieve to catch the seeds and combine the juice with the capers, shallot or onion, lemon juice, olive oil, salt, pepper, mint and parsley. Stir well, then add the fish.
Marinate in the refrigerator for 1 hour, stirring every so often.
Serve in ramekins decorated with lemon zest, cold or at room-temperature.