Pan Bagnat with Fennel

Name: Pan Bagnat with Fennel
Ingredients: 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; 1 12-long ciabatta loaf, halved lengthwise; 3 hard-boiled large eggs, peeled, sliced; 2 cups arugula