The most common way to refer to this person without sounding unnatural would be "Hello Sir/Ma'am". Normally we don't say the word chef or cook when asking this person a question. If you have any more questions feel free to ask me them!

Maybe worth noting that in Britain - and perhaps generally outside North America, although I could be wrong - "Sir" and "Madam / Ma'am" have almost completely fallen out of use. You would just be polite and not call him anything, or ask him what his name is and call him by that. The only common uses of "Sir / Madam" now that I can think of are in schools, and to begin letters, as in "Dear Sir..."

Damon is correct, but I would like to add that "Chef" can be used as a title if they are in fact a chef, and also if you know their last name. An example of this is the celebrity chef, Gordon Ramsay. On his TV programs, he is often called "Chef Ramsay". Example: "I'll be interested in knowing Chef Ramsay's opinion is about this dish." If you do not know if the cook is indeed a chef to give this title, and you do not know their last name, then as Damon suggested, you would just address them as Sir/Madam.