MELT butter in large saucepan on medium heat. Sprinkle with flour, garlic powder and salt. Cook and stir 1 minute. Slowly whisk in milk, beer and mustard until smooth. Stirring constantly, cook 5 minutes or until sauce starts to thicken. Gradually whisk in cheeses until melted and smooth.
TRANSFER cheese mixture to warm fondue pot or slow cooker to keep warm, if desired. Serve with assorted dippers, such as cubed or sliced crusty bread, cut-up fresh veggies, pretzels or chips.