The Brisket Burger is absolutely delicious and always cooked perfectly to order. It comes with candied red wine onions, truffle brie, and fries that you should be dipping in the provided truffle aioli. By default it comes with a brioche bun but you should instead ask for it to be served on the potato bun as the potato bun provides the perfect meat-to-bread ratio without the sweetness of the brioche.
I've had nearly every burger in town and Onyx's Brisket Burger is the second best burger in all of metro Phoenix.