A Rotterdam Casserole is the act of farting under your bedclothes, usually a duvet or something nice and thick, followed by your sticking your head (or your partners) under the sheets to allow the savouring of such a wonderful smell - it also gives your partner a chance to guess what you may have had for lunch :)
Man (to his wife): Have you ever had a rotterdam casserole before?
Wife: No, what's it like?
Man: Here, I'll make you one... (Farts loudly under the duvet and lifts it up over his wife's head)
Wife: Oh, you dirty bastard!