Preheat the oven to Gas 4/ 180°C.
Boil a large pot of water and cook potatoes until soft, for approx. 15 minutes.
Take the frozen vegetables out of the freezer. While the potatoes are cooking, simmer one cup of the milk in a large pot.
Add onion, season with pepper and add the stock cube.
Cut the fish into 6 pieces, add to the milk mixture and cook on the low heat for about 10 minutes until the fish flakes with a fork.
Remove the fish from the pot (retain this pot and liquid) and place in an 8 x 8 inch ovenproof dish.
Divide the fish into smaller chunks.
Sprinkle parsley over the fish and then add the raw or frozen vegetables.
Add the cornflour, butter and 1/2 cup of milk to the pan in which the fish was cooked. Stir frequently with a whisk and cook on the hob on a low-medium heat for approx. 2-3 minutes until thickened.
Pour the sauce over fish and vegetables.
Drain water from potatoes then add 1/2 cup of milk and butter and mash until smooth.
Spread the mashed potato on top of the fish and bake in the oven for 20 minutes until the vegetables are cooked.