1In shallow glass or plastic dish or resealable food-storage plastic bag, mix all marinade ingredients. Pierce beef with fork several times on both sides. Add beef and onions to marinade; turn to coat with marinade. Cover dish or seal bag; refrigerate, turning beef occasionally, at least 8 hours but no longer than 24 hours.
2Heat gas or charcoal grill. Remove beef onions from marinade; reserve marinade. Place beef and onions on grill. Cover grill; cook over from medium heat 15 to 20 minutes for medium doneness, brushing occasionally with marinade and turning once. Discard any remaining marinade.
3Cut beef across grain into thin slices. Sprinkle beef with feta and parsley; serve with onion slices.