Combine the butter and flour in a medium heavy saucepan and cook, stirring, to form a light blond roux. Whisk in the milk and bring to a boil. Reduce the heat to a simmer and cook until the sauce is thickened slightly.

Stir in the grated cheese until smooth and melted. (If sauce is grainy, purée it in the blender until smooth)

In a separate saucepan, sauté shallot and garlic in the oil until soft. Add fresh thyme and beer and cook until beer has reduced and thickened.

Add the beer mixture to cheese dip base and mix until incorporated. Season with salt and pepper to taste. Serve sauce hot, with croustades or warm pretzels.