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Melt the butter in a medium-sized saucepan over medium heat.
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Once the butter is melted, add the flour all at once and stir to make a roux. (It will look like thick paste).
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Once it is smooth and bubbly, slowly pour in the milk while stirring quickly or using a whisk to make a smooth sauce.
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Continue to cook over medium heat, stirring constantly, until the sauce thickens.
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Add the lemon juice, mustard, salt, paprika, and pepper.
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Bring the sauce barely to a boil and then remove from the heat.
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Pour over the chicken, ham, and cheese.