Gosh, I don't think of spanakopita as being at all obscure, you can get them in any deli and I've made them several times. But maybe being a veggie they're more of a staple for me than you.

I had a similar experience in a restaurant in San Francisco a few years ago. Only, sad to relate, all the several things I didn't know were actually the names of vegetables, rather than obscure treatments and sauces. The only one I remember was "ramps" which is a kind of wild leek.