Gooey melted cheese with a bitter kick is the comfort food of your dreams.
Rub inside of a medium saucepan with both halves of the garlic and discard.
Add butter and melt over low heat. Add flour and salt and cook, stirring constantly with a wooden spoon, until smooth and slightly thickened, 2 minutes. Switch to a whisk. Slowly pour in the beer, whisking constantly, and bring to a boil.
Reduce heat to low. Add cheese one handful at a time, whisking after each addition, until cheese is melted and smooth. Increase heat to medium and cook until small bubbles form around the edges. Remove from heat and stir in mustard. Serve with soft pretzels, toasted baguette, or vegetables for dipping. Fondue can be reheated over low heat, stirring until smooth, or transfer to a slow cooker on low, and keep warm for up to 1 hour.