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1
In a large pot of boiling water, blanch the chard until wilted, about 1 minute. Drain the chard, squeeze it dry, and chop. Be sure to remove any excess moisture.
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2
Heat the oil in the pot. Add the leeks and a pinch of salt. Cover and cook over medium to medium low heat, stirring, until tender, 7 minutes. You know your stove -- if it cooks hot make it medium low, if it cooks low, make it medium. Uncover, add the garlic, and cook, stirring, until fragrant. (about 2 minutes). Add the chard, season with salt to taste and remove from the heat.
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3
Preheat the oven to 425 degrees. Butter or grease a 2 qt casserole dish.
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4
In a large saucepan, melt the butter. Stir in the flour over moderate heat to form a paste. Gradually whisk 1/3 of the milk and cook, whisking, until the mixture starts to thicken. Repeat two more times with the remaining milk.
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5
Bring the sauce to a boil, whisking constantly. Reduce the heat to low and cook, whisking often, until thickened and no floury taste remains, about 10-15 minutes. Whisk in the cheeses and the nutmeg. season with salt and pepper.
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6
Combine the sauce and leeks/chard mixture. Season with salt and pepper to taste.
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7
Transfer to the prepared casserole dish. Bake in the upper third of the oven for 25 minutes. Let rest for 10 minutes before serving to prevent burning your mouth.
