Add the cheese and stout to the saucepan and place over low heat. Be careful: if the heat is too high, the cheese will separate. When the cheese begins to melt, add the spring onions, mustard and egg yolk, and cook for a couple of minutes, stirring constantly, until the mixture thickens and comes away from the sides of the saucepan. Season with Worcestershire sauce, remove from the heat and leave to cool.