Just because this is low calorie doesn't mean it's lacking in flavour or portion size. I've played around with this recipe and it'll deliver four man-sized portions and plenty of flavour for less than 500 calories per serving! I achieved most of this simply by substituting half of the potato for swede (the swede gives the mash a nice colour too) and using a low calorie cheese spread instead of Red Leicester or Cheddar.
You can use any combination of fish you like or that is in season or on offer, but some fish like salmon may raise the calories (though the calories in fish are out weighed by the nutrition anyway). The calorie saving trick with the mash and swede can also be used on a shepherd's pie.
I have worked about 1800 calories in the entire dish, which makes for just over 450 calories per portion if serving four.