* 75g (2 1/2oz) butter, plus extra for greasing
* 1 large leek, trimmed and cut into 5mm (1⁄4in) thick slices
* 50g (1 3⁄4oz) plain flour
* 600ml hot, full-fat milk
* 1 1⁄2 tbsp chopped fresh dill
* finely grated rind of 1/2 lemon
* salt and freshly ground black pepper
* 400g (14oz) hake fillet, skinned and cut into 2.5cm (1in) cubes
* 400g (14oz) salmon fillet, skinned and cut into 2.5cm (1in) cubes
* 200g (7oz) fresh spinach
For the topping
* 800g (13⁄4lb) new potatoes, such as Charlotte, scrubbed
* 2 tbsp olive oil, plus 2 tsp for drizzling
1.7–2 litre baking dish, about 25 x 18cm (10 x 7in) and 7.5cm (3in) deep.
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1. Grease the baking dish with butter. Melt 50g of the butter in a large pan over a medium heat. Add the leek and fry for three minutes or until softened but not browned. Stir in the flour and cook, stirring, for one to two minutes.
2. Remove from the heat and gradually pour in the hot milk. Return to a medium heat and stir until boiling, thickened, and smooth.
3. Stir in the dill, lemon rind, and some salt and pepper. Add the hake and salmon to the sauce. Cook over a low heat for two minutes, stirring gently twice, just to start cooking the fish.
4. Pour the fish mixture into the buttered baking dish and set aside to cool. You can prepare up to this point one day ahead and refrigerate overnight.
5. Melt the remaining butter in a large, non-stick, deep-sided frying pan or sauté pan over a medium heat. Add the spinach and cook, stirring, for one and a half to two minutes or until it wilts.
6. Drain thoroughly in a colander, pressing down with the back of a wooden spoon to extract excess moisture. Roughly chop and set aside. Preheat the oven to 200ºC (fan 180ºC/400ºF/Gas 6).
7. Meanwhile, make the topping: put the potatoes in a large pan of cold salted water and bring to the boil. Cover and simmer for about 15 minutes or until tender. Drain well.
8. Return the potatoes to the pan and, using a fork, break the potatoes down into rough, chunky pieces. Stir in the olive oil and some salt and pepper.
9. Scatter the spinach on top of the cooled fish mixture, then spoon the potatoes over the spinach layer. Drizzle over the remaining two teaspoons of oil and bake for 30–40 minutes or until the pie is bubbling at the edges and the topping is golden and crispy.
Mary Berry's keys to perfection
1. Make a smooth creamy filling: the leek should look wilted before you stir in the flour. However, you don’t need to cook it completely, as it will finish cooking in the sauce. Stir the flour constantly for one to two minutes, making sure it is combined thoroughly with the butter. Don’t let the butter or flour brown. You’re aiming for a pale gold paste, or “roux”.
Adding hot milk will blend into the roux more easily than cold milk and helps to prevent the sauce from turning lumpy. Bring the milk to a gentle simmer, then remove it from the heat. Don't simmer for too long or it will start to reduce in the pan. Add it gradually to the roux, stirring continuously.
Allowing time for the fish and sauce to cool down will provide a firmer base for when you spoon the potatoes over the top, ensuring they won’t sink in
2. Let the filling cool down: after adding the dill, lemon rind, and seasoning, add the fish to the sauce. Don’t stir too much, or the fish may start to break up; you want to keep it in chunky pieces. Just give a couple of gentle turns using a wooden spoon so the fish is well coated with the white sauce.
Let the fish and sauce cool in the baking dish. This will provide a firmer base for when you spoon the potatoes over the top, so they won’t sink in. Spoon the spinach over the sauce before adding the topping. The spinach must be well drained so the sauce doesn’t get watery.
Crush the potatoes into irregular chunky pieces but don't break them down too much
3. Make a crispy topping: put the whole potatoes in a pan of cold water (if very large, cut them in half) and start the timing once the water comes to the boil. Reduce the heat slightly and let the water simmer gently. They will take about 15 minutes to cook. To test if the potatoes are done, insert the tip of a sharp knife; it should go in easily.
To crush the potatoes, use a fork to break each one into several chunky pieces. You don’t want to break them down too much, and they don’t need to be completely uniform in size. By crushing them the oil will be absorbed into the potato pieces, which will improve their flavour and make them golden when baked.
More from Mary Berry:
Mary Berry Cooks the Perfect, published by DK, £25, dk.com. Also available to download on iBooks.