1. Melt two tablespoons of the butter in a medium skillet over medium heat.

2. Add the flour and whisk briskly to form a paste. Continue cooking, whisking constantly, for one or two minutes, until the nutty flour scent dissipates.

3. Slowly add the beer, whisking vigorously to prevent lumps.  Continue to cook, whisking, until the sauce thickens.

4. Blend in the mustard, Worcestershire sauce, salt, and pepper.

5. Stir in the grated cheese and cook until melted and smooth.  Set the sauce aside.

6. Toast the bread, then slice each piece in half to form triangles.  Butter with the remaining four tablespoons of butter.

7. Arrange the bread in a single layer on a baking sheet and pour the cheese mixture over.

8. Place under the broiler for about one minute, until the cheese bubbles and turns a pale, golden brown.

9. Serve immediately.