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1
In a large pot of boiling water, blanch the chard in batches until wilted, about 1 minute.
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2
Drain the chard, squeeze dry and chop it.
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3
Heat the oil in the pot.
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4
Add the leeks and a pinch of salt.
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5
Cover and cook over moderately low heat, stirring, until tender, 7 minutes.
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6
Uncover, add the garlic and cook, stirring, until fragrant, 2 minutes.
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7
Add the chard, season with salt and remove from the heat.
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8
Preheat the oven to 425.
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9
Butter a 10-by-15-inch baking dish.
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10
In a large saucepan, melt the butter.
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11
Stir in the flour over moderate heat to form a paste.
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12
Gradually whisk in one-third of the milk and cook, whisking, until the mixture starts to thicken.
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13
Repeat two more times with the remaining milk.
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14
Bring the sauce to a boil, whisking constantly.
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15
Reduce the heat to low and cook, whisking often, until thickened and no floury taste remains, 15 minutes.
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16
Whisk in the cheeses and the nutmeg; season with salt and pepper.
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17
Mix the sauce into the leeks and chard.
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18
Season with salt and pepper.
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19
Transfer the mixture to the prepared baking dish.
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20
Bake in the upper third of the oven for about 25 minutes, until bubbling and golden brown on top.
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21
Let rest for at least 10 minutes before serving.
