Simmer a cup of milk in a pot and add the small onion and season with pepper.

Cut the fish into large pieces and place in the milk mixture and simmer for 10 minutes or until the fish flakes with a fork.

Remove the fish from the pot (saving aside the liquid) and place the fish pieces in an oven proof dish divided into smaller pieces

Add on top the frozen peas and corn.

Make a roux using ½ your quantity of melted butter and mixing it with 1 tbs of flour.

Pour the sauce over the fish and vegetables.
Mash the potatoes when cooked using the remains of milk and butter until soft and smooth.
Spread mash potatoes over the fish and bake in the oven for 20 mins.

Serve.