225 g assorted mushrooms, such as oyster, horse, shiitake, trimmed and quartered

675 g cod or 675 g haddock fillets, skinned and diced

600 ml milk, low-fat is fine, boiling

For the Topping

• 900 g floury potatoes, peeled and quartered
• 25 g unsalted butter
• 150 ml milk, low-fat is fine
• salt, to taste
• fresh ground black pepper, to taste
• grated nutmeg, to taste

For the Sauce

• 50 g unsalted butter
• 1 medium onion, chopped
• 1/2 celery rib, chopped
• 50 g plain flour
• 10 ml lemon juice
• 2 tablespoons fresh parsley, chopped

Directions:

1. 1
   Preheat the oven to 200°C/400°F/Gas 6 and butter an ovenproof dish, scatter the mushrooms in the bottom, add the fish and season with salt and pepper, to taste; pour the boiling milk over the top, cover and cook in the oven for 20 minutes.
2. 2
   While the mushrooms and fish are cooking, cover the potatoes with cold water; add a good pinch of salt and boil for 20 minutes; drain and mash with the butter and milk, and season to taste.
3. 3
   Using a slotted spoon, transfer the fish and mushrooms to a 1.5 litre (11/2 quart American; 1.5 litre Canadian) baking dish. Pour the poaching liquid into a jug and set aside.
4. 4
   To make the sauce: Melt the butter in a saucepan, add the onion and celery and sauté until soft but not browned; stir in the flour, then remove the pan from the heat; slowly add the reserved liquid to the pan, stirring until it has been absorbed; then return the pan to the heat, stir and simmer to until the sauce thickens; add the lemon juice and parsley, season to taste, then add the sauce to the baking dish, using the sauce to cover the fish and the mushrooms.
5. 5
   Top with the mashed potato and place the baking dish in the oven for 30-40 minutes until the potatoes are golden