Place the canned tuna (from dolphin-friendly catch) in a colander and drain.
Rinse and dry the tomatoes. Cut large ones into quarters, small ones into halves and fill into a salad bowl.
Clean and rinse the spring onions and cut into thin rings. Add to the tomatoes form.
Roughly chop the tuna with a fork, add to the vegetables in the baking dish.
Mix the vinegar, oil, salt and pepper with a whisk to make a creamy marinade, pour over the lettuce, mix well and leave to marinate for about 10 minutes.
Preparation : about 25 min.