Keys to Perfection

Make a smooth, creamy filling

1 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 1-2 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’.

2 I like to add hot milk when making a white sauce, as I find it blends 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. If you keep it simmering for too long, it will start to reduce in the pan. Add it gradually to the roux, stirring continuously.

3 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 filling cool down

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.

Make a crispy topping

1 Put the whole potatoes in a pan of cold water (if very large, cut them in half) and start 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.

2 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.

Carefully add the topping

Spoon the potatoes quite loosely over the top of the filling, rather than packing them down, so the broken edges can become crisp.