Is it possible to obtain the recipe for the casserole??

As far as I'm aware, no, only she can make them.

Not that I've seen unless you mod.

Just don't try to play merchant come up with a scheme to start a casserole delivery service, collect 56+ rad scorpion stingers worth 34+ caps and make casseroles worth 6'ish caps.
Barter 100, buisness sense 0.