1Rub inside of 3-quart saucepan with cut sides of garlic; discard garlic. Add butter to saucepan; melt over low heat. Stir in flour; cook 2 minutes, stirring constantly. Stir in beer. Increase heat to medium-high; heat to boiling. Boil 2 to 3 minutes, stirring constantly, until mixture is thick and smooth.
2Reduce heat to low. Add cheese, about 1/2 cup at a time, stirring until cheese is melted and mixture is smooth.
3Transfer cheese mixture to fondue pot. Immediately place fondue pot over flame. Serve with dippers. Garnish dip with onions.