Pour the bottle of beer into a medium sized pan and bring to a simmer.
Turn the heat down to medium and add the cubed cream cheese. Using a whisk, work it into the beer until it's completely melted. It's normal for the sauce looks a little broken at this point.
Turn the heat down to low and add the cheese a handful at a time. Whisk each handful in before adding another. The sauce will go through stages and look like it's not coming together, but just keep whisking it over low heat. After about 4-5 minutes, it will become very smooth and thick. Do not let it boil once you've added the cheese, or the sauce will break.
When all of the cheese is melted and the sauce is smooth, remove it from the heat and taste for seasoning. I usually add a pinch of salt and a healthy amount of black pepper, but you can add more Sriracha, mustard, or whatever other spices you like.
Serve immediately with green onions sprinkled on top for garnish, if desired.