Toast the slices of bread on each side using a toaster or broiler.
In a small saucepan, melt the butter over medium heat.
Whisk in the flour and cook until golden.
Slowly whisk in the beer and continue to whisk until the mixture thickens.
Mix in the mustard, Worcestershire sauce, salt, and pepper to season.
Reduce heat to low and add the shredded cheese. Whisk until melted and smooth. Adjust seasonings to taste and remove from heat.
Place the toasted slices of bread on a rimmed baking sheet. Cover with the cheese sauce, using a spatula or spoon to gently push the sauce to cover the whole surface.
Place under the broiler until the cheese is bubbly and turns golden in spots.
Serve immediately, topped with fresh herbs if desired.