In a saucepan over medium heat, melt the butter.
Stir in the flour to create a roux, cooking for 1-2 minutes until lightly golden.
Add Dijon mustard, Worcestershire sauce, smoked paprika, and black pepper to the roux, stirring to combine.
Gradually pour in the ale or beer, whisking continuously to avoid lumps.
Once the mixture is smooth and thickened, reduce the heat to low.
Gradually add the grated cheddar cheese, stirring until the cheese is fully melted and the sauce is smooth.
Toast the slices of bread until golden brown.
Pour the cheese sauce generously over the toasted bread.
Optionally, place the Welsh Rarebit under a broiler for a minute or two until the top is bubbly and lightly browned.
Serve immediately, savoring the rich and savory flavors of Classic Welsh Rarebit.
