Open-Faced Tuna Sandwiches with Arugula and Sweet-Pickle Mayonnaise

INGREDIENTS

• 1 (1-pound) loaf ciabatta bread, halved horizontally (see Cook's Note)
• Kosher salt and freshly ground black pepper
• 4 (4 to 6-ounce) albacore tuna steaks
• 2 cups arugula, thinly sliced
• 4 slices sharp white Cheddar
• 1/2 cup finely diced sweet pickles (about 2 pickles)
• Olive oil, for drizzling, plus 1/4 cup
• 4 teaspoons herbs de Provence
• 1/2 cup mayonnaise