Mix oil and garlic in bowl. Hollow out inside of both bread halves, leaving 1/2-inch-thick sides. Brush inside of bread halves with garlic oil. Spread tuna evenly over bottom half. Top with onion, then tomatoes, pressing down to compact. Sprinkle anchovies and capers over, pressing to adhere. Layer peppers over, then olives. Top with remaining bread half; press together. Wrap bread tightly in plastic wrap. Refrigerate at least 2 hours and up to 1 day. Cut bread crosswise into six 3-inch-wide pieces.