Right I cook my fish first in foil in the oven say 180 electric just for 15 mins add either milk or butter.
I get potatoes on first so they will be ready
whilst fish is cooking I make the sauce and whilst that is cooking I am boiling/steaming the vegetables,when all is cooked I chuck in oven-wear dish with sauce oh and prawns add them as they are (I am presuming they are cooked pink) mix together and add the mash on top could grate some cheese over too. Put in oven for about 15 mins on a lower setting say 150-160
Or you can make the sauce and after fish has cooked chuck in with sauce and mix up.