This is one of those stories that becomes funnier as time goes by and provides continuing entertainment as you ponder the perfect response.
My current suggestion: "Asparagus? No, I believe it was a Brussel sprouts casserole. Would you like the recipe?"
My favorite snappy answer so far!
However, my response would be a variation of Allyson's. I would give her a well-bred flat affect, followed by a raised eyebrow and a very proper "I beg your pardon" in a tone designed to make her dig herself in deeper. It's a bit mean-spirited, but people like this usually only get broken of such behavior when they end up making world-class fools of themselves.