Barbecued Steak Sandwiches

This recipe is from Cooking Light. It's so yummy, and perfect for summer! (I double this for our family)

INGREDIENTS:
1 (1-pound) flank steak, trimmed (I use sirloin)
1/2 teaspoon salt
1/4 teaspoon freshly ground black pepper
4 (1/4-inch-thick) slices red onion
1/4 cup ketchup
2 tablespoons light brown sugar (We like it with less sugar-1 T)
1 tablespoon cider vinegar
1 tablespoon Worcestershire sauce
1/4 teaspoon chipotle chile powder (optional)
4 (3-ounce) hoagie rolls, split (Wal-mart's hard rolls are good)

Heat a grill pan over medium-high heat. Coat pan with cooking spray or oil. Sprinkle steak evenly with salt and pepper. Add steak to pan; cook 6 minutes on each side or until desired degree of doneness. Remove steak from pan; let stand 10 minutes. Add onion to pan; cook 3 minutes on each side or until lightly browned. Remove from pan; cool.
Combine ketchup and next 4 ingredients (through chipotle chile powder) in a large microwave-safe bowl, stirring well. Microwave at HIGH 1 minute or until thoroughly heated. Cut steak diagonally across grain into thin slices. Add steak to ketchup mixture; toss to coat. Divide steak mixture evenly among bottom halves of rolls; top each serving with one onion slice. Place top half of roll on each sandwich.
4 servings. This is great to make ahead and keep warm in crock pot.