Wash the celery, cut into thin slices and blanch in boiling salted water. Wash parsley, pluck leaves from the stalks and chop coarsely. Cut cucumber into slices and mix with celery and parsley.
Drain the tuna, cut it into small pieces and put it on the salad. For the vinaigrette, clean, wash, quarter and seed the tomatoes. Finely dice the flesh. Mix tomatoes, capers, lemon juice and oil.
Season with salt, pepper and sugar. Pour over the salad. Arrange with fresh farmhouse bread and serve garnished with lemon wheels.