Prep dishes? He transformed this dish (9,3)
I believe the answer is:
shepherds pie
'this dish' is the definition.
(I've seen this before)
'prep dishes? he transformed' is the wordplay.
'transformed' is an anagram indicator.
'prep'+'dishes'+'he'='prepdisheshe'
'prepdisheshe' with letters rearranged gives 'SHEPHERDS PIE'.
(Other definitions for shepherds pie that I've seen before include "Food" , "Mashed potato and minced meat, baked" , "Minced meat topped with mashed potato" , "Meat and mashed potato dish" , "Meat-and-potato dish" .)