French Fish Pie

Step 1: Place the potatoes in a large saucepan and cover with cold water. Bring to a boil, then simmer for 25 minutes, until very soft. You can start making the sauce while the potatoes cook.

Step 2: Preheat the oven to 200°C / 390°F (180°C fan). Place a foil-lined baking tray at the bottom of the oven to catch any filling that bubbles over.

Step 3: Pat the fish pieces dry with paper towels and sprinkle salt over all sides. Set aside.

Step 4: Heat the oil and butter in a 26 cm / 10-inch cast-iron skillet over medium-high heat. Add the onion, fennel, leek and garlic. Cook for 5 minutes, stirring regularly, until softened and light golden.

Step 5: Add the flour and stir for 30 seconds. Pour in the white wine; it will simmer rapidly at first. Mix it in until the mixture looks pasty. Slowly pour in the hot milk and cream while stirring. Add the salt and pepper. Bring to a simmer and cook for 1 minute, until thickened. The sauce should be quite thick, coat the back of a spoon and then run gently down it. Set aside.

Step 6: Finish the mashed potato as described in the mashed potato section.

Step 7: Stir the dill into the sauce. Add the fish pieces, nestling them gently into the sauce so they are mostly submerged. Using a spatula, carefully move the fish around just enough to coat it in the sauce. Smooth the surface.

Step 8: Pipe mounds of mashed potato swirls over the surface. The swirls shown have a 4.5 cm base and are about 4 cm tall; fill the gaps with smaller swirls or blobs and use all the mash. Spray lightly with olive oil.

Step 9: Bake for 35 minutes, rotating the pie halfway through, until golden on top. If it is not golden enough, place it under the grill for a few minutes.

Step 10: Rest for 10 minutes before serving so the pie cools slightly and the sauce thickens further.

Step 11: Drain the potatoes well. Pass them through a potato ricer, or return them to the pot and mash well with a potato masher. Do not use a stick blender, which can overwork the starch and make the mash gluey instead of fluffy. Add the butter, salt and white pepper, and stir until combined. Add the hot milk and stir until smooth. The mash should be creamy but not loose, so it holds its shape when piped.

Step 12: Transfer the mash to a piping bag fitted with a star tip.