Fill a large pot with salted water, place the zucchini into the pot and boil over medium heat for about 5 minutes to soften. Remove the zucchini, slice in half the long way and allow to cool. Scoop out the flesh from the zucchini halves, leaving a 1cm shell. Set aside the scooped out flesh in a bowl.
Preheat oven to 180 degrees C. Lightly grease a baking dish.
Place the zucchini flesh into a bowl and mash well. Mix in the tuna, onion, tomato, bread crumbs, egg, salt and black pepper. Lightly stuff the zucchini shells with the tuna mixture. Drizzle about 1/2 teaspoon of olive oil over each stuffed zucchini half.
Bake in the preheated oven until the tops are slightly browned, 20 to 25 minutes.