Cut the tomatoes crosswise in halves, gently remove the seeds with a spoon. Leave the tuna to drain, flake it with a fork. Mix it with the chopped onion and the white wine vinegar, season with salt and pepper and fill the tomato halves.
Cut the avocado lengthwise in halves, remove the stone and carefully lift the pulp from the peel, using a tablespoon. Cut the avocado in thin slices and arrange them fan-like on the Crispbread. Place the stuffed tomatoes on top of the avocado and garnish with parsley.
For a weight-conscious variant use salt water tuna.
Finely chop the tomato flesh and mix it to the tuna.
When placed at an ambient temperature and wrapped in newspaper, avocado continue to mature.