FOR each serving: Place 1 piece tuna between 2 sheets of waxed paper; pound to 1/8-inch thickness. Use 2-inch round cutter to cut 4 rounds from tuna; arrange on serving plate. Season with pinch of salt and 2 turns of pepper; drizzle with 1 Tbsp. bistro sauce.
DRIZZLE 1 Tbsp. of the remaining bistro sauce and 2 tsp. balsamic vinaigrette around edge of plate. Sprinkle tuna with 1/8 tsp. sesame seeds; top with 1/4 cup arugula.
CUT 1 baguette piece into 4 thin lengthwise slices. Brush with oil; broil until toasted, turning once. Arrange on platter.