For the mashed topping
- 1 kg (6 ½) cups floury potatoes, peeled and cut into chunks
- 120 g (1/2 cup) unsalted butter, at room temperature
- 120 ml whole milk, warmed
- 2 medium egg yolks
- Freshly ground black pepper
For the filling
- 1 tbs olive oil
- 1 leek, sliced, washed, and drained
- 3 tbs unsalted butter
- 3 tbs plain flour, or cornflour
- 500 ml whole milk
- 100 ml double cream
- 300 g (2 cups) white fish fillets, eg. haddock, cod, or pollock, pin-boned, cubed
- 300 g (2 cups) fresh or frozen prawns, thawed if frozen
- 200 g (1 1/3 cups) frozen peas, thawed
- 1 small handful dill, chopped
For the mashed topping: Cook the potatoes in a large saucepan of salted, boiling water until tender to the tip of a knife, about 20-25 minutes.
Drain well and return them to the pan they were cooked in, cooking over a medium-low heat until dried out, about 2-3 minutes.
Add the butter and mash well until smooth before adding the warmed milk and egg yolks, mashing until smooth. Season to taste with plenty of salt and pepper. Cover and set aside.
For the filling: Preheat the oven to 180°C (160° fan) | 350F | gas 4.
Heat the olive oil in a frying pan set over a medium heat. Add the leek and a pinch of salt, sweating until softened, about 6-8 minutes. Set off the heat.
Melt the butter in a large saucepan set over a medium heat. Whisk in the flour and cook until blonde in colour, about 2-3 minutes.
Whisk in the milk and cream in a slow, steady stream until thickened. Bring to a simmer and cook steadily for 6-7 minutes, whisking frequently.
Fold in the white fish, prawns, peas, and sweated leek. Season to taste with salt and pepper as needed before spooning into an oval baking dish.
Spread the mashed potato over the top of the filling, flicking it to peaks with a spoon. Scatter the dill on top.
Bake for about 45 minutes until browned on top. Remove from the oven and let stand for at least 5 minutes before serving.