Start by boiling the potatoes in water for 10-15 minutes until tender.
Meanwhile, place the fish and bay leaves in a shallow pan, pour over 250ml of the milk and cook over a low heat for 10 minutes.
While the fish is cooking, start to make the sauce. Melt 25g butter or spread in a non-stick saucepan and stir in the flour. Carefully stir in 250ml milk to make a white sauce. Continue to stir until the sauce thickens.
When the fish is cooked and flakes easily, lift carefully from the pan and place in a serving dish. Cover and keep warm. Strain the poaching milk into the white sauce and heat through, stirring well.
Add the peas and sweetcorn to a pan of boiling water (unsalted), cook for 3-4 minutes, then drain.
Drain the potatoes and mash with 50ml milk and 10g butter
Just before serving, stir the parsley into the sauce and pour over the fish fillets.
Serve the fish at once with the mash and vegetables.