- Preheat grill to medium-high and grease grates well. Pat steaks dry with paper towel, and season with salt and pepper.
- Grill steak for 4 to 5 minutes per side for medium rare, or cook to your liking. Let steak rest for 10 minutes.
- Toss red onion slices with olive oil. Grill onions for 2 to 3 minutes per side, or until tender. Once steak has rested, slice thinly against the grain.
- Grill baguette cut side down for 30 seconds, or until lightly marked. Spread tapenade on half of the baguette slices and place Mozzarellissima slices on the remaining. Grill, covered, for 1 minute, or until cheese starts to melt.
- Sandwich steak, onion, tomato and arugula between toasted baguette with tapenade and cheese. Serve immediately.