Preheat your grill (broiler) to high and place a rack about 6–8 inches from the heat. Lightly toast slices of the Crusty Bread so they are just crisp — this keeps them from going soggy under the sauce.
Grate the Mature Cheddar Cheese and set most of it aside for melting and a little for sprinkling on top.
In a medium saucepan, melt the Butter over medium heat. Stir in the Plain Flour and cook for 1–2 minutes to make a pale roux, stirring constantly so it doesn't brown.
Gradually whisk in the Ale until smooth, then whisk in the Milk a little at a time until the sauce is silky and begins to thicken.
Stir in the Worcestershire Sauce, English Mustard Powder, a pinch of Cayenne Pepper, and season with Salt and freshly ground Black Pepper. Taste and adjust the seasoning — the sauce should be punchy but balanced.
Remove the pan from the heat and stir in most of the grated Mature Cheddar Cheese until melted and glossy. If the sauce is too stiff, loosen with a splash more milk or ale.
Temper the Egg Yolk by whisking a small spoonful of the hot sauce into the yolk, then quickly whisk the warmed yolk back into the sauce to enrich it — do this off the heat to avoid scrambling.
Pile or spoon the cheese sauce generously onto the toasted Crusty Bread slices, scatter the reserved grated Mature Cheddar Cheese over the top, and place the slices under the hot grill until the topping is bubbling and deeply golden, about 2–4 minutes — watch closely so it doesn't burn.
Remove from the grill, let sit for a minute to set, then serve immediately while still oozy and warm. A final crack of Black Pepper is lovely if desired.
