Pretty much all the ingredient amounts depend on the size of the dish your using and personal preference. I made this for my mother in law and myself, she took half home with her i ate the rest after she left...Its quite moreish.
You can always add more or less cheese to this or even use different types of fish.
Peel the potatoe's and chop into small pieces. Put them on to boil with a little salt in the water.
Whilst the potatoes are boiling make the sauce. Add the butter, milk, garlic powder, onion powder, salt and pepper to a saucepan and put on a medium heat. stir this occasionally. Add the majority of the cheese (save some to go on top).
When all the cheese has melted mix the cornflour with the small amount of water stir it well and add to the sauce. Turn the heat up for a short time until the sauce thickens then take off the heat.
In a medium sized pyrex dish layer the 2 different types of fish and the prawns.
Add the peas and sweetcorn. Then mix together using your hands (a bit messy i know but its the best way).
When the potato's are soft enough drain and mash them adding a little butter and salt.
Pour the sauce over the fish and veg. Only use enough to cover it all as too much will make your mash sink into the sauce.
If there is any sauce left over you can mix a few tablespoons into the mash.
Top the fish and veg with the mash. The best way to do this is put small amounts all over then smooth with the back of a fork to even it out. If you put big dollops of mash on to the mixture it will sink.
Sprinkle the remaining cheese on top.
Place in the oven at 150 Degrees and cook until the top goes golden brown. Which is about 20-30 mins, but keep an eye on it as you dont want it to over cook.