Cook the tuna belly in a little water. Cool and flake.
Heat an omelet pan until very hot (I used a cast iron fying pan to minimize sticking. Teflon-coated frying pans will do the trick just as well). Melt the butter or margarine.
Cook the carrot cubes for 1 minute. Add the potato cubes. Cook for another minute. Add the raisins, onions and garlic. Cook until the carrots and potatoes are done. Remove with a slotted spoon and transfer to a plate to cool slightly.
Meanwhile, stir in the flaked tuna into the beaten eggs. Season with salt and pepper. Stir in the cooked vegetables and blend. Divide the mixture into two portions.
Reheat the pan. Pour in one portion of the mixture and cook, covered, over medium-low heat for 3 minutes. Turn off fire but do not remove the cover. The heat will continue cooking the eggs without burning it. The frittata will be fully set and ready to eat in five minutes.