Heat 1 tablespoon of butter in a stove-top friendly dutch oven or large saucepan over medium-low heat.
Once the butter is melted, add the sliced mushrooms and the 1/4 cup of beer.
Simmer for approx. 8 minutes, or until all of the butter and beer has been absorbed by the mushrooms.
Season with a bit of kosher salt and ground black pepper.
Transfer to a bowl and set aside.
Prepare the pasta noodles as per the instructions on the packaging.
In the same vessel you cooked the mushrooms in, melt 2 tablespoons of butter.
Gradually whisk in 2 tablespoons of flour. Continue to whisk until a thick roux forms.
While whisking, add the milk.
Bring the heat up a bit so the milk comes to a boil, and whisk until the roux has dissolved into the milk, thickening the sauce.
While whisking, slowly add 1/2 cup of beer.
Allow to simmer until the sauce thickens again.
Add the shredded cheeses and stir until the cheese melts.
If the sauce is too thick, you can gradually add more milk as you whisk until you reach a desired consistency.
Season with kosher salt and ground black pepper to taste.
Transfer the mushrooms to the cheese sauce and stir.
Drain the pasta once it's done. Don't rinse.
Carefully dump the pasta into the cheese sauce and stir to coat.
For a slightly crispy top, place the mac and cheese under a broiler (on high) for a few minutes. Time varies since all ovens are different, so keep the oven light on and keep a very close eye on it.
Notes
Beer must be at room temperature. Cold beer may break the sauce.