First place potatoes in a large saucepan, cover with cold water. Bring to a boil, then simmer 25 minutes until very soft.
Preheat oven to 200°C / 390°F (180°C fan).
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 cast-iron skillet over medium-high heat. Add onion, fennel, leek and garlic. Cook 5 minutes until softened.
Make sauce – Add flour, stir for 30 seconds. Pour in white wine. Slowly pour in the HOT milk and cream while stirring. Add salt and pepper. Bring to a simmer, cook 1 minute until thickened.
Finish filling – Stir in dill. Add the fish pieces to the sauce, gently nestling them in. Smooth the surface.
Pipe mounds of mashed potato swirls on the surface. Spray lightly with olive oil.
Bake – Bake 35 minutes, rotating halfway, until golden on top. Rest for 10 minutes before serving.
