1. Heat a grill pan over medium-high heat. Sprinkle steak with salt and pepper. Add steak to pan, and cook 2 1/2 minutes on each side or until desired degree of doneness. Remove the steak from pan, and let stand 5 minutes. Cut steak across grain into thin slices.
2. Combine mayonnaise and pesto, stirring until well blended. Spread mayonnaise mixture evenly over cut sides of bread. Layer bottom half of bread with arugula, red onion, steak, and tomato; top with top half of bread. Cut sandwich diagonally into 4 equal pieces.
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