- 1/2 fennel bulb, very thinly sliced
- 1/2 cup mixed pitted olives, chopped
- 1/4 cup olive oil
- 3 tablespoons white wine vinegar
- 2 tablespoons chopped drained capers
- 1/2 teaspoon crushed red pepper flakes
- Kosher salt, freshly ground pepper
- 1 Persian or 1/4 English hothouse cucumber, thinly sliced
- 2 (6-7-ounce) cans olive oil–packed tuna, drained
- Kosher salt, freshly ground pepper
- 1 12″-long ciabatta loaf, halved lengthwise
- 3 hard-boiled large eggs, peeled, sliced
- 2 cups arugula
- Toss fennel, olives, oil, vinegar, capers, and red pepper flakes in an airtight container or resealable plastic bag; season with salt and pepper.
- Fennel-olive mixture can be made 2 days ahead. Cover and chill.
- Toss fennel-olive mixture with cucumber and tuna in a medium bowl; season with salt and pepper. Let sit, tossing occasionally, 10 minutes.
- Scoop out inside from top half of bread; discard (or save for a snack!). Layer bottom half of bread with tuna mixture, then top with eggs and arugula. Close up sandwich.
- Place a large heavy skillet or pot on top of sandwich to weigh down and let sit at room temperature, turning once, until evenly pressed, about 2 hours (if making sandwich at home, or storing in a cooler while you hike, wrap it up tightly in plastic wrap before you press it).
- Slice pan bagnat into wedges just before serving.