Top-ranked recipe named "Barbecued Flank Steak Sandwiches"
Heat a grill pan over medium-high heat. Coat pan with cooking spray. 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.