Combine first 6 ingredients in a medium bowl. Add tuna; stir gently.
Hollow out top and bottom halves of bread, leaving a 1-inch-thick shell; reserve torn bread for another use. Place bread, cut sides up, on a baking sheet. Broil 1 minute or until toasted.
Spoon tuna mixture evenly into bread shells; top each with 2 tomato slices and 2 tablespoons cheese. Broil 2 minutes or until cheese melts. Sprinkle 1 teaspoon green onionson each sandwich, if desired.