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Place potatoes in a large pot of water and boil until soft.
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Meanwhile, in another large pot, melt butter on medium-low heat.
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Once melted, add flour to create a roux - stir for 30 seconds.
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Add milk, salt, pepper, and onion salt.
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Stir soup until thick. {It helps to turn up the heat a bit at this point. Be sure to keep stirring it though, so the milk doesn't burn.}
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Drain potatoes and add to soup.
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Add sour cream and mix well.
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Serve while warm.