Outdoor kitchen in Yellowstone (bears)?

You will get the rules when you get to the park; you can cook outside, eat outside, but cannot leave anything unattended, even a fork or water bottle. The rangers will ticket you (and, of course, it's for your safety and that of the bears...as they say in Yosemite "a fed bear is a dead bear").
If you go to nps.gov you can probably find a contact e-mail for yellowstone to get an official and current answer before your visit; I have had luck contacting Yosemite with a specific question.