My wife and I had brunch at a restaurant today where all of the hot dishes are served buffet style in the kitchen itself. You simply walk into the kitchen, grab your plate and serve yourself. (As background, we had dinner at this restaurant a few months ago, and we met the chef at our table.) Today, the chef himself was working the carving station. When we approached, I said, "Chef, we met a few months ago blah blah blah" and we exchanged pleasantries.
In this instance, is it proper to call him "Chef" or should that be used by the staff only? Otherwise, I'm not really sure what I would have called him.