In large, shallow nonmetal dish or resealable food-storage plastic bag, mix all marinade ingredients. Make pocket in side of flank steak by cutting lengthwise almost but not completely through opposite side. Add steak to marinade; turn to coat. Cover dish or seal bag; refrigerate at least 4 hours or overnight to marinate.
In 8-inch skillet, heat oil over medium heat. Add onion; cook 2 minutes, stirring occasionally, until tender. Stir in garlic and parsley; cook and stir 1 minute. Add bread crumbs and lemon peel; cook and stir 1 minute. Remove from heat.
Heat grill. Remove steak from marinade; discard marinade. Fill pocket in steak with stuffing. Secure opening with toothpicks.
Place steak on gas grill over medium heat; cover grill. Cook 12 to 20 minutes, turning once, until steak is desired doneness. Cut steak across grain into 1/2-inch-thick slices.