Drain mozzarella, artichokes and tuna separately. Wash and slice the tomatoes. Peel and halve the onion and cut into thin strips. Wash the basil, shake dry and pluck the leaves.
Dice mozzarella. Halve the artichokes. Roughly pluck the tuna.
Mix vinegar, salt, pepper and 1 pinch of sugar. Fold in 4 tablespoons of oil. Mix with the prepared ingredients.
Cut the bread horizontally. Spread the tomato mixture on the bread halves. Sprinkle with 1 tablespoon of oil and season again with salt and pepper if necessary. Serve immediately.