Wash mushrooms and pat dry. Apply gentle pressure with your thumb to push out the stems (set them aside for another recipe or stock).
Put bread pieces into a medium bowl, add a little warm water to cover for a minute. Then drain thoroughly and squeeze excess water out of the bread. Add tomato, walnuts, onion, olives, vinegar and garlic. Mix well and season to taste.
Stuff a generous amount of the mixture into each mushroom cap. To serve, line a small platter with lettuce leaves, arrange the mushrooms on top, and garnish each with a tiny sprig of fresh parsley. You can chill them or enjoy as is.