Weirdly, it was when we went to Genghis Khan's in Manchester.
I was still a food-fuss then, and the thought of eating Mongolian food terrified me. But then I realised during that meal that Mongolian food (and probably by extension, other food) is just the same as our food - it's still the same meat, same vegetables etc. but just prepared differently and with different sauces.
So I decided to always try stuff. If I didn't like it, fair enough, but at least I'd given it a go - and I also told myself to try things again fairly regularly in case my tastes change, and that's how I can now tolerate some cheeses rather than outright hating everything with cheese in the name
There's only one exception to this rule - mushrooms. Can't stand the texture of 'em, never have been able to. The smell of them cooking and in my food makes me gag. Jem occasionally chops up mushrooms really small in some meals, and I can deal with that as it avoids both problems - but eating them normally can fuck right off.
So yeah. My food fussiness almost certainly stemmed from growing up in a household that only ever ate British staples - roasts etc. - because my Dad was (and is) a tremendously fussy git food-wise, and my Mum wasn't much better.