Pressed Tuna Sandwich (Pan Bagnat)

Combine onion, 2 tablespoons vinegar and water in a small bowl. Marinate, stirring occasionally, for 10 minutes.

Meanwhile, cut loaf in half horizontally. Pull out the soft inner bread from each half, leaving a 1/2-inch border. (Save the inner bread for another use, such as making breadcrumbs.)

Whisk the remaining 3 tablespoons vinegar, oil, mustard, pepper and salt in a large bowl. Drain the onion (discard the marinade) and add to the dressing along with tuna, arugula, cucumber, radishes, tomatoes and olives. Toss to coat.

Stuff the tuna mixture into the bottom half of the loaf. Top with eggs, basil and the top of the loaf. Wrap tightly in plastic wrap. Place in the refrigerator and weight down with a large cast-iron skillet or large pot for 2 hours or overnight. Cut into 8 slices.