Clean the leek, wash it and cut it into rings. Blanch briefly in boiling salted water. Rinse and drain the beans. Cut cabanossi into thin slices. For the marinade, peel and roughly chop the onion.
Mix vinegar and mustard. Season with salt, pepper and sugar. Whip oil into it, add onions. Mix with the prepared salad ingredients. Cover and leave to stand for at least 1/2 hour. Then season again and serve.