Step 1Halve the baguette lengthwise and scoop out a little of the interior of both sides with your fingers.
Step 2In a small bowl, whisk the vinegar, lemon juice, salt and pepper until combined. Whisk in the olive oil until emulsified and set aside.
Step 3Coarsely chop the olives and capers, then combine in a small bowl with the minced garlic and set aside.
Step 4Fill the bottom part of the baguette with the olive mixture, spreading it evenly across the hollowed-out baguette. Layer the tuna over the olives, then, in even layers, add the potatoes, eggs and tomatoes. Drizzle the vinaigrette over the sandwich, then add the onions and the arugula, pressing down on the contents as you go. Top with the other baguette half and wrap the sandwich tightly with plastic. Refrigerate overnight, weighted with a cutting board or a plate topped with some cans or bottles.
Step 5The next day, take the sandwich out of the refrigerator in the morning and cut into sixths. Wrap individually and pack for lunch: the sandwiches are best when they've been sitting at room temperature for a couple of hours.