Step 1

In a medium saucepan, melt the butter over medium-low heat. Add the onion and cook, stirring, until softened, 4 to 6 minutes. Gradually whisk in the flour and cook, whisking constantly, until lightly browned, about 1 minute.

Step 2

Gradually whisk in the beer and half and half. Cook, whisking constantly, until the sauce is thickened and just begins to bubble, 5 to 7 minutes. Reduce the heat to low and gradually add the cheeses, whisking constantly, until melted and combined. Remove from the heat.

Step 3

Whisk in the Dijon mustard and the Worcestershire sauce. Garnish with chives and black pepper, if you like. Serve with pretzels; re-warm gently as needed.