Melt the margarine in a pan.
Add the flour and mix in with a wooden spoon.
Cook on a gentle heat for a few minutes without coloring.
Gradually add the cold milk and mix to a smooth paste.
Allow to simmer for a few minutes: add grated cheese.
Allow to melt slowly over a gentle heat until the mixture is smooth.
Add the yolk to the hot mixture and stir in and quickly remove from the heat.
Meanwhile, in a separate pan boil the beer and allow it to reduce to half a tbspn.
Add to the mixture with the other seasonings.
Allow the mixture to cool and spread on the four rectangles of buttered toast.
Place on a baking sheet and brown gently under the salamander and serve.
NB; Cheese contains a lot of protein and may become stringy and tough if heated for too long or at a high temperature.