The deal is tied to their sponsorship of the college football Outback Bowl played yesterday. Depending on which team won, the chain was prepared to give away either Coconut Shrimp (if Northwestern won) or Bloomin' Onions (if Tennessee won). As you may have guessed (or watched), Tennessee won so Bloomin' Onions it is.
To take advantage of the offer just mention "Outback Bowl" to your server and buy anything off the menu. The offer is good for one Bloomin' Onion with any purchase on January 2, 2016, while supplies last.