• 3 tbsp plain flour
• 1 tsp Dijon mustard
• 700g potatoes, peeled and chopped into chunks
• 60ml milk
• 3 tbsp chives, snipped
• 300ml milk
• 500g smoked coley (skin and bones removed, chopped into bite-sized chunks)
• 30g butter
• 1 onion, finely chopped
• 150g button mushrooms, sliced
• 1 tbsp fresh parsley, chopped
• 1 tbsp lemon juice
• Salt and black pepper
• 60g Cheddar, grated
• For the leek and chive mash:
• 40g butter
• 1 leek, washed, trimmed and sliced

1. Preheat the oven to 180˚C/160˚C fan/gas mark 4.
2. Place the milk in a large saucepan and add the fish. Bring the milk to a simmer over a medium heat and poach the fish for 7-8 minutes. Use a slotted spoon to remove the fish to a plate. Cover and keep warm. Strain the milk into a jug and set aside.
3. Wipe out the saucepan and return to a medium heat. Melt the butter in the pan, then add the onion and mushrooms and cook for 4-5 minutes until softened.
4. Remove the pan from the heat and sprinkle in the flour. Gradually add the reserved poaching milk, stirring continuously until the mixture forms a thick sauce.
5. Add the mustard, parsley and lemon juice. Season with salt and black pepper and stir to combine. Gently stir in the Cheddar and poached fish, then transfer everything into a baking dish.
6. To make the leek and chive mash, melt 20g of butter in a pan over a medium heat and cook the leek for 5-6 minutes until soft. Season with black pepper, then remove from the heat.
7. Place the potatoes in a large pot and cover with water. Add two teaspoons of salt and bring to the boil. Cook for about 20 minutes until the potatoes are soft. Drain well, then add the milk (or cream) and the remaining 20g of butter. Mash the potatoes until smooth, then stir in the softened leek along with the chives and some salt and black pepper.
8. Spoon the mashed potato over the top of the fish mixture. Place the pie in the oven for 20-25 minutes until the mash is lightly golden on top and the sauce is bubbling around the edges. Serve immediately.