Skin and trim the fillets. Cut all of the vegetables into 5 mm/¼ inch dice. Butter a tray and scatter the vegetables over the bottom. Lightly season the fish and lay on top of the vegetables, then pour over the beer. Cover with buttered paper or a lid and place in the oven for about 12-15 minutes. When cooked, remove the fish and vegetables, cover and keep warm.
Pour the liquor into a saucepan, add the fish stock and reduce over a high heat by at least three-quarters. Pour in the cream, return to the boil and reduce until it just starts to thicken.