In a saucepan over medium heat, melt the butter. Add the flour and mustard powder, stirring into a smooth roux. Cook for 1 minute to eliminate the raw flour taste.
Gradually whisk in the ale to avoid lumps. Stir in Worcestershire sauce and bring to a gentle simmer.
Lower the heat and slowly add the grated cheese, stirring until fully melted and smooth. Season with salt and pepper. If using the egg yolk, stir it in off the heat for extra richness.
Place your toasted bread slices on a baking tray. Generously spoon the cheese mixture over each slice.
Broil for 2–3 minutes, or until the top is bubbling and golden.
Serve hot with a pint of ale and maybe some pickled relish for contrast.