Green’s Dictionary of Slang

manhole cover n.2

1. as food [joc. ‘resemblance’].

(a) bread pudding.

[UK]Partridge DSUE (8th edn) 721: since ca. 1945.

(b) (US short order) hot-cakes.

[US]Ames (IA) Daily Trib. 11 Dec. 1/1: Or if you [...] tell the waitress your order, and she yells ‘Manhole covers,’ don’t get up and leave. She’s only sending in your order of hot-cakes.

2. see under manhole n.