How do you-all pronounce FLAN (the fruit/custard tart)?

Well here in SA we would say "flan" for the fruity dessert. I might say "flahn" to refer to the Spanish dessert (what most English speakers call creme caramel) but even that is bordering on the pretentious. Maybe your waitron (cringe) had heard someone speak of the latter and assumed that it was the sophisticated pronunciation of the former.

It is my experience that people waiting tables (especially in SA where it is not seen as a real job) are pathetically clueless about food and certainly not up to the challenges of the words on their menus.