Arrange the fish - in fillets or cubed, with the skin removed - in a dish.
Prepare the marinade using 2/3 lime juice to 1/3 olive oil.
Drench the fish fillets in this marinade, and then add the tomato and salt.
Enjoy with slices of toast.
Note: You can add other ingredients depending on what you feel like at the time: onions, shallots, peeled and deseeded cucumber, fresh pepper, etc