Follow these steps carefully for the perfect Creamy Fish Pie:

1. Prepare the Mashed Potato Topping: Peel the potatoes and cut them into even-sized chunks (about 1.5-2 inches). Place them in a large saucepan and cover generously with cold salted water. Bring to a boil, then reduce the heat and simmer for 15-20 minutes, or until the potatoes are very tender when pierced with a fork. Be careful not to over-boil, as they can become waterlogged.
2. Drain and Mash: Once tender, drain the potatoes thoroughly in a colander, letting them steam dry for a couple of minutes. This step is crucial for removing excess moisture, resulting in fluffier mash. Return the potatoes to the warm pan (off the heat).
3. Mash to Perfection: Add the room temperature butter, warmed milk or cream, salt, and pepper to the potatoes. Mash vigorously using a potato masher or ricer until smooth and creamy. Avoid using a food processor, as this can make the mash gluey. If incorporating cheese into the mash, stir it in now until melted. Taste and adjust seasoning if necessary. Cover the mash and set aside, keeping it warm.
4. Poach the Fish: While the potatoes are cooking, prepare the fish. Pour the 750ml of whole milk into a wide, shallow pan or deep frying pan. Add the bay leaf, peppercorns, and halved onion. Bring the milk just to a simmer over medium-low heat – do not let it boil rapidly.
5. Cook the Fish Gently: Add the mixed fish chunks (cod, haddock, salmon) to the simmering milk. Ensure the fish is mostly submerged. Reduce the heat to low and gently poach for 6-8 minutes, or until the fish is just cooked through and beginning to flake easily. It should still hold its shape. Carefully remove the fish pieces from the milk using a slotted spoon and place them onto a plate. Reserve the poaching milk – this flavourful liquid is the secret to a delicious sauce! Discard the bay leaf, peppercorns, and onion halves.
6. Make the Roux: In a large saucepan (you can use the one you cooked the potatoes in after washing it), melt the 75g of unsalted butter over medium heat. Once melted and foaming gently, whisk in the plain flour. Cook the mixture (this is called a roux) for 1-2 minutes, stirring constantly, until it forms a smooth paste and smells slightly nutty. Cooking the flour prevents a raw flour taste in your sauce.
7. Create the Béchamel Sauce: Gradually add the reserved warm poaching milk to the roux, a ladleful at a time, whisking continuously after each addition to prevent lumps. Ensure the mixture is smooth before adding more milk. Once all the milk is incorporated, bring the sauce to a gentle simmer, still whisking.
8. Thicken and Season the Sauce: Let the sauce simmer gently for 5-8 minutes, stirring frequently (especially around the edges and bottom of the pan) to prevent sticking and allow it to thicken to a consistency similar to double cream (it should coat the back of a spoon). Stir in the Dijon or English mustard and a generous grating of fresh nutmeg. Taste the sauce and season generously with salt and freshly ground black pepper. Remember the smoked haddock will add saltiness, so taste before adding too much salt. If desired, stir in a splash of double cream now for extra richness.
9. Combine the Filling: Gently flake the poached fish into large, bite-sized chunks directly into the creamy sauce. Be careful not to break it up too much. Add the cooked king prawns (if using), frozen peas, quartered hard-boiled eggs (if using), chopped fresh parsley, and chopped fresh dill (if using). Gently fold everything together until just combined. You want distinct pieces of fish and egg in a luscious sauce.
10. Preheat the Oven & Assemble the Pie: Preheat your oven to 200°C (400°F, Gas Mark 6). Choose a large ovenproof dish (approximately 2-litre capacity, or a 23x33cm / 9×13 inch baking dish). Pour the creamy fish filling evenly into the dish.
11. Top with Mash: Spoon the prepared mashed potato evenly over the fish filling, starting from the edges and working inwards to create a seal. Be gentle so the mash doesn’t sink too much into the filling. Spread it out smoothly with the back of a spoon or a palette knife.
12. Create Texture: For a lovely crispy, textured topping, run a fork over the surface of the mash to create ridges. If you opted for extra cheese, sprinkle the grated Cheddar or Gruyere evenly over the top now.
13. Bake Until Golden: Place the fish pie dish on a baking tray (to catch any potential spills) and bake in the preheated oven for 30-40 minutes, or until the mashed potato topping is beautifully golden brown and crisp, and the filling is bubbling gently around the edges. If the top starts to brown too quickly, you can loosely tent it with foil for the remaining cooking time.
14. Rest Before Serving: Once baked, carefully remove the fish pie from the oven. Let it rest for 5-10 minutes before serving. This allows the filling to settle slightly, making it easier to serve and preventing lava-like temperatures! Garnish with a little extra chopped parsley if desired.