Add bacon to a large skillet over medium heat. Sauté 10 minutes until just crispy. Remove and place on a paper towel-lined plate. Cool and then chop bacon into rough pieces. Discard the bacon grease.
In a medium high-sided skillet or saucepan on medium heat, melt the butter. Add the onions and cook until soft, about 3-4 minutes.
Whisk in the mustard and then add the beer, scraping up the bits from the bottom of the pan. Increase the heat, boil, and cook for 5 minutes, stirring occasionally.
Add the cream cheese then reduce the heat to medium. Whisk the cream cheese into the beer mixture until the cheese melts and becomes creamy. Turn off the heat.
Set aside ¼ cup of the shredded cheddar. Add the rest of the cheddar cheese about 1 cup at a time to the beer mixture, stirring until the cheese melts and is smooth. Repeat until all of the cheese has been added.
Stir in ¾ of the bacon, saving the rest for the top of the dip.
Spray a 1-quart baking dish with cooking spray. Pour the beer cheese into the dish and top with the remaining ¼ cup cheddar cheese and the rest of the crumbled bacon.
Bake for 20-25 minutes or until bubbling. Serve warm with your favorite bread.
