Melt the butter in a medium saucepan over medium heat. Stir in the garlic and cook until fragrant, 1-2 minutes. Whisk in the flour.
Remove the pan from heat, then whisk in 2 tablespoons of the milk until smooth. Whisk in 2 more tablespoons of milk. Add the rest of the milk in three parts, whisking until smooth in between additions.
Stir in the beer, dijon, cayenne and salt. Return the pan to the heat and stir until thick and bubbling, about 2-3 minutes. Turn off the heat and remove the pan from it.
Add the cheese a handful at a time, whisking after each addition until melted through and smooth. Add more salt and cayenne to taste. Garnish with a pinch of cayenne and chopped parsley.
Serve with soft pretzels, bread, chips or veggies.