I normally buy salmon, river cobbler (cheaper than haddock or cod) and prawns
I bake the salmon and river cobbler in th oven for about 15-20 mins, put fish on tin foil and add pepper.
OR you can cook the fish in milk, just depends
Whilst the fish is cooking I boil potatoes
Male white sauce - melt a large knob of butter, add about 2-3 heaped tablespoons of flour cook mixture on a low heat for a few minutes stirring occasionally. Take off the heat and add a little milk, whisk, put back on heat and wait to thicken. Once thickened add more milk and repeat until you have enough. Season with salt and pepper
Mash potatoes and add butter and milk to them
Take the fish out, should be cooked by then! Flake it, i normally just get a fork and mash it a little bit
Put the fish into a ovenproof dish, stir in the prawns (don't cook before hand), add the white sauce, enough so that the fish is coated and not dry you can add peas at this stage to the fish mixture or serve on the side
Top with mashed potato and a sprinkle of grated cheese, put i oven for 15-20 mins