In a medium bowl, toss the shredded cheese with the flour and set aside.
Add the beer to a medium pot, and heat until it just starts to bubble (do not let it boil). Add one cup of the cheese, stirring in a figure-eight pattern until fully melted. Repeat with the remaining cups of cheese until it's fully incorporated.
Stir in the mustard, garlic powder, and pepper. Transfer to a warm fondue pot, or keep it warm over very low heat until you're ready to serve. If you're eating it right away, you can also pour it into a ramekin or cast iron. Make sure to stir it frequently so it doesn't congeal. Enjoy!
* Percent Daily Values are based on a 2,000 calorie diet. Your daily value may be higher or lower depending on your calorie needs.