And before that, http://singletrackworld.com/forum/topic/pie-definitions-pleae/page/2#post-5600301
It’s really quite simple.
Can you pick it up in your hands and carry it to your mouth? If you cannot (for any reason other than “it’s a really big one”) then it is not, by any sensible definition, a pie. A pie is self-sustaining and stands alone, proud. It does not require third party support from some party-sized casserole dish.
“Cottage pie” and their ilk get special exemption from this definition as they are not “pies” of a “cottage” type, but rather “cottage pie” is its full name. It does not, after all, actually contain cottages(*). It’s a bit like the “slow worm” which, despite its name, is not actually a worm (or a snake either for that matter).
(* – Except when served in East Lancashire football stadiums, perhaps)