In a mixing bowl, stir together the salsa, Lipton® Recipe Secrets® Onion Soup Mix, lime juice and sugar until blended. Reserve 1 cup of marinade and set aside.
Place the flank steak into a zipper-top plastic bag, and pour the remaining marinade over the steak. Seal the bag and marinate for at least 8 hours or overnight, turning occasionally.
Remove the steak from the marinade, discard marinade. Grill or broil the steak, brushing frequently with reserved marinade for 5 minutes on one side. Turn and grill another 3 to 5 minutes, or until the meat is grilled to desired doneness.
Remove from the grill, and cut across the grain, on an angle into thin strips. Serve, if desired, with rice, beans, tortillas, and condiments.