Combine first 3 ingredients in a medium microwave-safe bowl; microwave at HIGH 2 minutes or until boiling. Stir in onion. Let stand at room temperature for 30 minutes.
Preheat grill to medium-high heat.
Combine mayonnaise and next 4 ingredients (through garlic).
Rub steak evenly with oil; sprinkle with salt and pepper. Place steak on grill rack; grill 5 minutes on each side or until desired degree of doneness. Remove from grill; let stand 5 minutes. Cut steak across the grain into thin slices.
Cut baguette in half lengthwise. Hollow out top and bottom halves of bread, leaving a 1/2-inch-thick shell; reserve torn bread for another use. Place bread, cut sides down, on grill rack; grill 1 minute or until toasted.
Drain onion mixture; discard liquid. Arrange steak evenly over bottom half of baguette; top evenly with onion and arugula. Spread mayonnaise mixture over cut side of top baguette half; place on sandwich. Cut into 4 pieces.