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1
In a small saucepan, melt butter over medium-high heat (do not allow it to brown). Add flour and whisk to form a paste. Continue to cook, stirring, until raw flour scent is gone, about 1 minute. Whisking constantly, add milk in a thin, steady stream, or in increments of a couple of tablespoons at a time, whisking thoroughly and getting into all corners of the pan to maintain a homogeneous texture. Sauce will initially become very thick, then get very thin once all the milk is added.
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2
Heat, stirring, until sauce comes to a simmer and begins to thicken slightly. Reduce heat to low and cook, stirring, until sauce is thick enough to coat the back of a wooden spoon, about 3 minutes.
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3
Season with salt and pepper. Add nutmeg to taste, if using. If any lumps form, simply whisk thoroughly to remove them and smooth out sauce, or use a
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4
or
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5
if lumps are particularly large or tough. Working in 2 or 3 additions, whisk in cheese, incorporating each addition just as the one before it is on the verge of fully melting; make sure to keep the heat low after the cheese goes in, as too much heat can cause the sauce to break. Remove from heat.
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6
Use sauce right away, or press a piece of plastic wrap over surface of sauce to prevent a skin from forming and keep warm until ready to use. Sauce can be cooled and stored in a sealed container in the refrigerator for several days. Reheat very gently on the stovetop or in the microwave before use.
