Out at a restaurant? Want to feel falsely superior to your fellow diners? Ask leading questions about the owners of restaurants until you goad someone into saying restauranteur. Don’t worry; if you’re persistent enough someone’s bound to say it. Then — and this is where you get to indulge your inner blowhard — show the fool who said restauranteur up by pointing out that it’s properly restaurateur.
The backstory to this is pretty cute. Restaurant is derived from the substantive use of the Latin present participle restaurer, which means “to restore”. So restaurant originally came from something like “place of restoration”, and the purveyor of this restoration was a “restorer” (restaurateur), not a “restaurant-er”. It’s sort of like how you have messenger and message, but not *messenge.
This was news to me when I read about it yesterday, but it seems that it’s actually fairly well-known. Google shows 3 million hits for restaurateur and only 250,000 for restauranteur, and if you search for the latter it suggests that you might really mean the former. In American English at least, restauranteur, restaurateur, and restauranter all seem pretty acceptable. But if you want to be properly safe, I’d advise you use the proper, older term restaurateur.
[Note: Please, please don't do this. If you think a good thing to do in a social situation is to correct people's grammar, I assure you that you are mistaken.]