When you're in Harlem... just kidding!
For good (albeit pricey) Italian food, Merlo La Salumeria is just up the street. www.merlochicago.com
If you want something less expensive and very close to the theater, Lou Malnati's on Wrightwood is right around the corner from the theater and their deep-dish pizza is among the best in town. Phone ahead with your pizza order (773.832.4030) and they'll have it ready for whatever time you specify; that way you won't have to wait 30-45 minutes while seated for your pizza to bake. www.loumalnatis.com
For diner fare, Clarke's has a location up the street. www.clarkesdiner.com
I had dinner at Merlo La Salumeria before seeing Million Dollar Quartet and it was fantastic. The food was great but the service was stellar. We arrived late for our reservation due to traffic and so were limited in the time we had for dinner. Our waiter was very accomodating, he gave us suggestions for ordering so we could get served more quickly. We even had time for dessert. We didn't feel overly rushed altho we had less than an hour for dinner. It's an upscale, a little pricey restaurant but for a really lovely dinner, I highly recommend it. Oh, and they make their own pasta.