Place cod in a single layer in a non stick pan, add the spring onions and milk and poach for about 10 minutes depending on the thickness of the fish.
Drain cod and reserve liquid and spring onions. Flake fish, remove any bones and place into a greased casserole dish with the spring onions and top with the boiled egg.
Melt butter in pan used for poaching fish (who needs more dirty dishes!?), stir in flour and cook over a low heat until mixture bubbles for a minute or so (don't brown), gradually add reserved poaching liquid stirring constantly until it boils and thickens, season with salt and pepper.
Pour over fish and eggs, cover with mashed potato and top with grated cheese.
Bake at 220 degrees Celsius for 20 minutes until browned on top.