To streamline your prep time, chop all the onions needed for the marinade and relish at the same time, and make the relish while the steak is marinating. The steak can marinate for as little as 30 minutes or up to 24 hours, if you prefer.
Trim fat from steak. Combine wine, onion, soy sauce, and garlic in a large zip-top plastic bag; add steak to bag. Seal and marinate in refrigerator 30 minutes, turning occasionally.
Remove steak from bag, and discard marinade. Sprinkle steak with salt and pepper. Place steak on grill rack coated with cooking spray; grill 5 minutes on each side or until desired degree of doneness. Cut steak diagonally across the grain into thin slices. Place bread slices on grill rack; grill 2 minutes on each side or until lightly browned. Rub 1 side of each bread slice with 1 garlic clove half. Serve bread with steak and Corn-Tomato Relish. Garnish with basil sprigs, if desired.