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1
Bring a pot of water to a boil. Wash the potatoes thoroughly. Peel if you want to (I didn't), and chop into 1-centimeter thick slices. Boil the potatoes for 10 minutes, or until tender but not falling apart. Drain them and set aside.
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2
Meanwhile, top and tail the leek, and remove the outermost layer. Slice it in half lengthwise, wash thoroughly, and then chop into 1-centimeter slices. Dice the onion.
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3
Heat the oven to 200u00b0C (Gas Mark 6/400u00b0F).
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4
In a large frying pan, heat the oil. Then add the leek and onion. Cook over a medium heat for 5-10 minutes, until the vegetables are soft, and then stir in the nutmeg. Season well with salt and black pepper.
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5
Push the vegetables to the edge of the pan, leaving a gap in the centre. Add the butter to the centre of the pan, and once it has melted, add the flour and stir (it doesn't matter if some of the leek gets caught up in the flour mixture). After about a minute, and once all of the dry flour has been incorporated, begin adding the milk bit at a time. Stir well before adding more, getting rid of any lumps of flour. Gradually incorporate the leek and onion back into the sauce until the mixture is creamy and well-combined.
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6
Transfer a third of the leek mixture into the bottom of a baking dish (I used a round one), and add the slices of potato in one or two even layers. Top with the remaining leek mixture. Pour over a dash more milk if the mixture seems too thick-otherwise, just top with the grated cheese and bake for around 20 minutes, until the cheese is melted and beginning to brown.
