Actually I kind of have a solution for this but I don't know if it will work for you too. I moved the fireplace downstairs when that annoying chef wouldn't stop making gruel.
The first time didn't work but the second time did work. (I know I'm kind of stubborn) The chef will then cook only when you ask him to for the quest. I think it's a matter of positioning but I can't remember exactly where I moved that fireplace.
Sorry, if you have the patience you could find out by yourselves but I think it isn't actually worth it