To prepare steak, combine first 7 ingredients in a large zip-top plastic bag. Add steak; seal and marinate in refrigerator for 2 hours, turning bag occasionally. Remove steak from bag, reserving marinade. Pour marinade into a microwave-safe bowl; microwave at high 2 minutes or until mixture comes to a boil. Set aside.
To prepare dressing, combine arugula and mayonnaise; set aside.
Place steak on a grill rack coated with cooking spray; grill 8 minutes on each side or until steak is medium-rare or desired degree of doneness. Remove steak from grill; cover and let stand 5 minutes. Place onion slices on grill rack; grill 4 minutes on each side, basting occasionally with reserved marinade. Place rolls on grill rack, cut sides down; grill 2 minutes or until lightly browned.
Cut steak diagonally across grain into thin slices. Spread 2 tablespoons dressing on bottom half of each roll. Divide steak, tomato, and onion evenly among bottom halves of rolls. Top with top halves of rolls.
Wine note: A great steak sandwich like this one deserves a rich, fleshy red wine--one that will stand up to the savoriness of the grilled onions and the sweet meatiness of the steak. Since this is a sandwich, the wine shouldn't break the bank. My top choice: a California merlot. The Chateau Souverain Merlot 2001 (Alexander Valley, California, $18) is smooth, with hints of blackberries, plum, sage, and mushrooms. -Karen MacNeil