It's not just any given Sunday, it's National Cheeseburger Day. On Sunday, Sept. 18 many restaurants will be offering up a juicy deal for the American staple, but in the West Loop, BomboBar (Bar Siena's walk-up window) will offerl: The BomboBurger. Partner and "Top Chef" contestant, Fabio Viviani, has launched a "secret menu item" with sharp American cheese, braised onions, dijonnaise, and garlic butter pickles, served between a bomboloni bun. Yes, the burger is served on an Italian hole-less doughnut bun. The burger will be available for $5 for a single and $7 for a double.