In a large saucepan, melt the butter over medium heat. Add the garlic and cook until fragrant, about 1 minute.
Stir in the flour and cook for 1 minute, stirring constantly.
Slowly add the beer, whisking constantly until the mixture is smooth. Bring to a simmer and cook for 3 minutes, stirring often.
Stir in the Worcestershire sauce, dry mustard, and cayenne pepper. Reduce the heat to low and stir in the cheeses a handful at a time, stirring until melted and smooth.
Transfer the fondue to a fondue pot or slow cooker and keep warm over low heat. Serve with vegetables and crusty bread.
Interesting Facts
The Irish stout beer gives this fondue a unique flavor.
This fondue is great for entertaining as it can be kept warm in a fondue pot or slow cooker.
The combination of cheddar and Gruyere cheeses makes for a creamy, flavorful fondue.