For years I thought it was ‘rarebit’, but I was persuaded otherwise by various reputable food writers. Anyway, refined cheese on toast with a few savoury additions is what it is.
Well in advance of serving, pour the Guinness into a small pan and simmer until reduced by half. Add the cream and simmer again until reduced by two thirds. Leave to cool.
Preheat the grill to medium. Add the cheese, egg yolk, Worcestershire sauce and mustard to the reduced Guinness and season with salt and pepper to taste.