Jill, I don't know about calling croquettes diner food. Around here, it was considered
an appropriate dish for a proper company dinner. It is pretty much a very thick
beurre blanc with protein added, along with herbs, minced onion and diced celery.
Our favorite was salmon. My mom formed the croquettes into a cone shape with a
rounded top. They hold together best and taste best when they are deep fried.
They were really, really good.

I have never seen them on a diner menu, and I have been in plenty of diners.

N.