1. Make Greek Marinade in shallow glass or plastic dish or resealable plastic food-storage bag. 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 and refrigerate, turning beef occasionally, at least 8 hours but no longer than 24 hours.
2. Heat coals or gas grill for direct heat. Remove beef from marinade; reserve marinade. Cover and grill beef and onions 4 to 5 inches from medium heat 15 to 20 minutes for medium doneness, brushing occasionally with marinade and turning once. Discard any remaining marinade.
3. Cut beef across grain into thin slices. Sprinkle beef with feta and parsley; serve with onion slices