Preheat the oven to 190°C/170°C fan oven/Gas Mark 5. Place the fish in a roasting tin. Add the bay leaf, peppercorns and onion, milk and wine and season. Cover and cook for 25 minutes or until the fish is cooked. Lift the fish with a slotted spoon and set aside. Increase the oven temperature to 200°C/180°C fan oven/Gas Mark 6. Strain the milk and reserve. Flake the fish into large pieces and place in a 2.3 litre (4 pint) ovenproof dish and season.

Melt the butter in a pan, add the flour and stir until smooth. Cook for 1 minute, then stir in the cream and reserved milk; season. Bring to the boil, stirring continuously, then simmer for 1 to 2 minutes, or until thick. Add the spring onions to the sauce and pour over the fish.

To make the watercress mash, mix everything together; season well. Spoon over the fish and cook for 40 minutes, or until piping hot.