Is mash cooked with one Japanese dish (7)
I believe the answer is:
sashimi
'japanese dish' is the definition.
(I've seen this before)
'is mash cooked with one' is the wordplay.
'cooked with' indicates anagramming the letters (letters cooked into a new form).
'one' becomes 'i' (Roman numeral).
'is'+'mash'='ismash'
'ismash' mixed up with 'i' makes 'SASHIMI'.
(Other definitions for sashimi that I've seen before include "dish from Japan" , "Japanese raw fish - I'm as his (anag)" , "Japanese raw fish dish - his aims (anag)" , "food in Japan" , "fish food?" .)