Melt the butter and whisk in flour. Constantly whisk for about four to six minutes to cook out the raw flour taste.
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.
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.