1. Cut the bread in half lengthwise. Using your fingers, hollow out each half of the bread, leaving a 1/2-inch shell. Save bread from inside loaf for another use.
2. In a small bowl, combine tuna and vinegar; set aside.
3. In another small bowl, stir together mayonnaise or salad dressing and anchovy paste. Spread the inside of each half of loaf with mayonnaise mixture. Spoon tuna mixture into the bottom half of bread.
4. Arrange tomato slices over tuna and season with pepper. Arrange onions over tomatoes and sprinkle with parsley. Top with remaining bread half.
5. Wrap loaf in foil and store in the refrigerator for 2 to 24 hours. To serve, cut into 8 slices.