Sauté onion, fennel, leek and garlic, add flour, wine, milk, cream, salt and pepper to make the sauce, stir in dill and add fish. Pipe on mashed potato. Spray olive oil. Bake for 35 minutes until golden. Rest 5 minutes, then serve.

First place potatoes in a large saucepan, cover with cold water. Bring to a boil, then simmer 25 minutes until very soft. You can start making the sauce while the potatoes are cooking.

Preheat oven to 200°C / 390°F (180°C fan). I highly recommend placing a baking tray lined with foil at the bottom of your oven to catch any bubbling filling that may leak during baking. Easier to clean!

Season fish - Pat fish pieces dry with paper towels and sprinkle all sides with salt. Set aside.

Sauté vegetables - Heat oil and butter in a 26cm / 10" cast-iron skillet over medium-high heat. Add onion, fennel, leek and garlic. Cook 5 minutes until softened and light golden. Stirring regularly. (Note 5)

Make sauce - Add flour, stir for 30 seconds. Pour in white wine. It will simmer rapidly at first, then mix it in, it will look pasty. Slowly pour in the HOT milk and cream while stirring. Add salt and pepper. Bring to a simmer, cook 1 minute until thickened. The sauce should be quite thick, fully coating the back of the spoon then gently run down it. Set aside.

Finish mashed potato - (see instructions below).

Finish filling - Stir in dill. Add the fish pieces to the sauce, gently nestling them in so they’re mostly submerged. Then, using a spatula, carefully stir and move them around just enough to coat them in the sauce. Smooth the surface.

Potato topping - Pipe mounds of mashed potato swirls on the surface. My swirls have a 4.5cm base (1.6"), about 4cm tall (1.4"), then I fill gaps with small swirls/blobs. Make sure to use all the mash! Spray lightly with olive oil. (Note 6)

Bake - Bake 35 minutes, rotating halfway, until golden on top. If it's not golden enough, place it under the grill for a few minutes.

Serve - Rest for 10 minutes before serving to cool slightly and let the sauce thicken a bit more.

Finish mash - Drain well. Pass through a potato ricer or return to the pot and mash well with a potato masher. Do not use a stick blender, it will overwork the starch, turning the mash gluey instead of fluffy. Add butter, salt and white pepper, stir until combined then add hot milk. Stir until smooth. It should be creamy but not loose (so it holds its shape when piped).

Piping bag - Transfer into a piping bag fitted with a star tip. (Note 7)