Melt the butter and whisk in flour. Constantly whisk for about four to six minutes to cook out the raw flour taste.
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Whisk in beer and milk. Continue to whisk to ensure lumps don't form. Whisk in Dijon mustard, nutmeg and Worcestershire and bring to a simmer. Do not boil.
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Stir in the softened mascarpone. When it's mostly incorporated, stir in the Gruyere and Gouda until it melts. Season with salt and pepper and serve with warm pretzels.
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