Brush steak all over with 1 1/2 tsp. oil; season. Grill over medium-high, turning once, until browned, 9 minutes for medium-rare. Transfer to cutting board; let rest 10 minutes. Brush cut sides of baguette with 6 tbsp. oil. Grill, cut sides down, 2 minutes. Thinly slice steak. Build sandwiches with tapenade, steak and arugula.