Clean, wash and quarter the tomato and remove the seeds. Cut the flesh into small cubes. Pour tuna onto a sieve, pull apart and drain. Peel and thinly slice the onion. Peel garlic and chop very finely. Melt the fat in a small pan. Add garlic, fry a little, put slices of bread in it and fry briefly from one side.
Remove from the pan. Wash the basil, carefully dab dry, pluck the leaves from the stalk and cut into fine strips. Mix tomato cubes, capers, onion, tuna and basil. Pour the mixture onto the toasted bread slices. Sprinkle with pepper. Serve garnished with lettuce leaves if desired