In a large re-sealable plastic bag, combine garlic, lime juice, salt and black pepper, to taste. Cut steaks into thin slices (freezing steaks slightly will make them easier to slice). Place in bag, cover and refrigerate for 1 hour.
Remove steaks from marinade and pat dry. In large deep skillet, heat 2 tablespoon oil over medium heat until just smoking. Add steak in batches, making sure not to overcrowd pan, and cook until just brown on both Side dish, 4 to 6 minutes. Add additional oil if necessary.
Place cooked steak on rimmed plate and cover to keep warm. After all steak has been cooked, add marinade and onion to skillet and cook until onion softens, 4 to 5 minutes. Pour sauce over steak, add parsley, if desired, and serve hot.