13g butter
13g plain flour
1/2 tsp ready-mixed English mustard
1/2 tsp Worcestershire sauce
100ml lager/pils or pale ale
130g grated cheddar
1 egg yolk
4 slices of sourdough/crumpets
Method
Melt the butter and make a roux with the flour. Add the mustard, beer and Worcestershire sauce and cook for 5 more minutes, fold in the cheese, stir until melted and take off the heat to cool a bit. Stir in the egg yolk and season.
Spread the mixture on the slices of sourdough/crumpets and grill under a medium heat/at a medium height under bubbling and starting to brown - 2 or 3 minutes. (Spare sauce can be stored for a few days.)