Combine soy sauce, scallions, ketchup, sesame oil, and garlic in a large resealable plastic bag. Add flank steak, seal, and refrigerate for 4 hours to overnight.

Preheat an outdoor grill for medium-high heat and lightly oil the grate.

Remove steak from bag, and discard marinade. Grill steak 6 minutes on each side for medium rare or until desired degree of doneness. Remove from grill. Tent with foil and let rest for 5 to 10 minutes. Cut diagonally across the grain into 1/4 to 1/2-inch thick slices.