1Preheat the oven to 400F.
2In a medium saucepan, heat the butter and flour over medium-high heat until the butter melts; whisk until smooth, then whisk in the milk and bring to a boil. Once the mixture bubbles, cook it for a full minute, stirring until it thickens, then turn the heat down to low and quickly whisk in the cheese, egg yolk, mustard and Worcestershire. Season with salt and pepper and whisk until melted and smooth.
3Toast your bread in a single layer on a baking sheet in the oven until golden. If you like, butter your toast. Pour the cheese sauce over top and turn the oven up to broil; run the rarebit under the broiler for a few minutes, until golden and blistered on top. Serve immediately, or use as a base for sloppy Joes.