Place flank steak in a heavy-duty zip-top plastic bag; pour barbecue sauce over meat, turning to coat. Cover and marinate at least 30 minutes.
Remove meat from marinade. Place marinade in a small saucepan; bring to a boil, and cook 1 minute. Remove from heat, and set aside.
Place meat on grill rack coated with cooking spray; cover and grill 6 to 8 minutes on each side or until a thermometer registers 145° (medium-rare). Transfer to a serving platter; let stand 2 to 5 minutes. Cut diagonally across grain into thin slices. Sprinkle with green onions. Serve with reserved marinade.