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1. In a medium saucepan, melt the butter over medium heat. Make sure it doesn’t brown.
2. Once the butter is melted, add the flour to the saucepan. Whisk continuously for about 1-2 minutes to create a roux. The mixture should be smooth and bubbly but not browned.
3. Gradually pour in the milk while whisking constantly to avoid lumps. Continue to whisk until the mixture thickens, about 3-5 minutes.
4. Once thickened, reduce the heat to low and add the grated cheddar cheese, stirring until melted and smooth.
5. Stir in the Dijon mustard, Worcestershire sauce, cayenne pepper, and season with salt and pepper to taste. Mix well to combine all the flavors.
6. Remove the saucepan from heat and let the cheese sauce cool slightly.
7. While the cheese sauce is cooling, preheat your broiler to high.
8. Place the slices of bread on a baking sheet. You can lightly toast them under the broiler for about 1-2 minutes on each side if you prefer a crunchier base.
9. Once toasted, remove the bread from the oven and set aside. If you prefer softer bread, you can skip this step.
10. In a small bowl, beat the egg. Once the cheese sauce has cooled slightly, stir in the beaten egg until fully incorporated. This will give the rarebit a rich texture.
11. Ensure the cheese sauce is not too hot when adding the egg, as this could cause the egg to scramble.
12. Check the seasoning of the cheese sauce and adjust with more salt, pepper, or cayenne if desired.
13. Take the toasted or un-toasted bread slices and place them on the baking sheet.
14. Generously spoon the cheese sauce mixture over each slice of bread, ensuring an even coating. You can pile it high for a more indulgent dish.
15. Sprinkle a little extra grated cheese on top if desired for a cheesy crust.
16. Place the baking sheet under the broiler. Keep a close eye on it, as the cheese can burn quickly.
17. Broil for about 3-5 minutes, or until the cheese is bubbly and golden brown. Rotate the baking sheet halfway through for even cooking.
18. Once done, remove from the oven and let it cool for a minute before serving.
19. Transfer the Irish rarebit to a serving platter or individual plates.
20. Garnish with freshly chopped parsley for a pop of color and added flavor.
21. Serve immediately while hot, alongside a simple salad or pickles for a delightful contrast.

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