Drain the tuna. Peel garlic and onion and dice finely. Mix lemon juice and olive oil. Wash basil, dab dry, pluck leaves from the stalks and cut into fine strips.
Add garlic, onion and basil to the marinade and season with salt and pepper. Pick up the tuna and sprinkle with the marinade. Leave to marinate for about 30 minutes. Spread the tuna on the ciabatta slices and serve with lettuce, tomato quarter, basil and a slice of lemon as desired.