*available at italian markets and specialty foods stores. if unavailable, puree 1/2 cup pitted, black, brine-covered olives (such as kalamata) in processor or blender.
Combine all tuna, capers, mayonnaise and fresh lemon juice in medium bowl. Season with pepper. Cut each bread loaf crosswise into 3 pieces, then halve each piece lengthwise. Pull out centers of bread pieces, leaving 1/2-inch-thick crusts. Spread olive paste on inside of each bread piece. Cover olive paste with generous amount of arugula. Spread 1/2 cup tuna mixture onto each bottom piece of bread. Top with sliced tomato and sliced onion, then with top pieces of bread. (Can be prepared 6 hours ahead.)