Pan Bagnat with Fennel

Name: Pan Bagnat with Fennel
Ingredients: ½ fennel bulb, very thinly sliced; ½ cup mixed pitted olives, chopped; ¼ cup olive oil; 3 tablespoons white wine vinegar; 2 tablespoons chopped drained capers; ½ teaspoon crushed red pepper flakes; Kosher salt and freshly ground black pepper; 1 Persian or ¼ English hothouse cucumber, thinly sliced; 2 6.7-ounce cans olive oil–packed tuna, drained; 1 12-inch-long ciabatta loaf, halved lengthwise; 3 hard-boiled large eggs, peeled, sliced; 2 cups arugula