Melt the butter in a medium pot over medium heat. Whisk in the flour and saute for 1 minute. Stir in the garlic and cook for about 1 minute. Whisk in the beer and mustard and cook, stirring, for a few minutes until the mixture begins to simmer and thicken. Whisk in the grated cheese a little at a time. Once all the cheese has been added, continue to cook, whisking, until the sauce is smooth and just beginning to bubble, another 5 to 7 minutes. Whisk in the heavy cream and season with salt and pepper. Transfer to serving bowl and serve with soft or hard pretzels.
If the cheese sauce seems grainy, strain it through a fine-mesh strainer right before serving