Pre-heat the oven to 200°C / 400°F / Gas Mark 6.
To make mashed potatoes, cook the potato pieces in a pan with boiling water. Simmer for 10-15 minutes depending on the size of your pieces, until the potato is soft. Drain and mash with a little of the milk. Use a spoonful of cream or half fat crème fraiche instead if you have any leftover. Add a twist of milled black pepper.
Meanwhile, cut fish fillets roughly into pieces and place in an ovenware dish. Pour over milk and bake fish, uncovered, whilst the potatoes are cooking. The fish is ready when it looks opaque and flakes easily with a fork.
Remove fish dish carefully from oven and drain/spoon all the hot milk into a saucepan. Set dish with fish to one side.
Add butter and flour to the pan of hot milk and, using a balloon whisk, whisk over a medium heat. It will look lumpy to start but persevere and you will have a thick creamy all-in-one sauce.
Bring to the boil to thicken and thoroughly cook the flour then remove from heat. Add sweetcorn and a twist of pepper and pour sauce over the fish.
Top with the mashed potato and sprinkle cheddar on top if wished.
Bake for 10-15minutes to make the potato topping golden. Alternatively finish off under a pre heated grill.
