Preheat your broiler on high. Arrange the bread slices on a baking sheet and toast them lightly under the broiler for about 1-2 minutes on each side, until golden brown. Keep an eye on them to prevent burning.

In a medium saucepan over medium heat, melt the butter. Once melted, whisk in the flour and cook for about 1 minute, stirring constantly to form a roux.

Gradually add the milk to the roux, whisking continuously until the mixture thickens and is smooth, about 3-5 minutes.

Remove the saucepan from heat and stir in the grated cheddar cheese, Worcestershire sauce, Dijon mustard, paprika, salt, and pepper. If using, mix in the egg yolk for added richness. Stir until the cheese is fully melted and the mixture is creamy.

Spoon the cheese mixture generously over the toasted bread slices, spreading it evenly.

Return the baking sheet to the broiler and cook for an additional 2-3 minutes, or until the cheese is bubbly and golden brown. Watch closely to avoid burning.

Remove from the oven and let cool for a minute. Garnish with chopped fresh parsley if desired, then serve immediately.