Melt butter in a medium saucepan over medium heat. Add flour and cook for 30 seconds while whisking.
Slowly whisk in the beer and then the milk. Cook and whisk until thickened (about 5-7 minutes). Whisk in the mustard, Worcestershire and salt until combined.
Remove from heat and gradually whisk in the shredded cheese a small amount at a time. Whisk until completely melted and lumps no longer remain. (If after 5 minutes of whisking all the cheese there are still lumps remaining, transfer to microwave safe dish and heat then whisk in 10 seconds intervals until lump-free.)
Transfer to bowl and serve immediately, or transfer to mini Crock Pot to keep warm.
Use freshly shredded cheese and not the bagged kind.
Shred the block of cheese while cold but then let the cheese come to room temperature before using in the recipe.
Refrigerate leftovers and reheat on stovetop or microwave, or serve cold.