UPDATE - we didn't find the diner you referring to (I don't think we did anyway) but we ate at the Delmonico Gourment Market on Lexington btwn 40 & 41st on Thursday, in the wells fargo bldg - EXCELLENT food at the hot bar. Some of the best baked salmon I've ever had eating out, including stuff I've order at fancy restaurants. I loaded my plate with that and a lot of other things for about $10 but my sweetie only spent $5; and everything was delicious.
(Weirdly enough their website only shows a 59th street location)

Alas, it was not open on Saturday for a return visit. I forgot the city operates differently on saturday.