In large zip-top bag, combine garlic, lime juice, salt and black pepper. Cut steaks into thin slices (leaving steaks slightly frozen will make them easier to slice), then place in bag, cover and refrigerate for 1 hour. Remove steak from marinade and pat dry. In large nonstick sauté pan, heat 2 tbsp. olive oil until simmering. Add steak in batches, making sure not to overcrowd pan, and cook on each side for 2 to 3 minutes, until just brown. 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 onions to sauté pan and cook until onion softens, about 4 to 5 minutes. Pour sauce over steak, add parsley, if desired, and serve immediately.