Start by melting the butter, then add the flour and mustard powder and cook for a minute or two.
Slowly add the milk and beer, an ounce or so at time, letting each addition of liquid fully and smoothly incorporate into the fatty flour and come up to temperature before adding the next
Once all the liquid has been added, crack some black pepper into the resulting sauce, and mix in a couple of dashes of Worcestershire
Slowly add the cheese, an ounce or so at a time, letting each addition melt fully, smoothly incorporate into the sauce, and come up to temperature before adding the next