Melt butter in a saucepan and gently cook leeks until tender.  Add mustard and season with salt and pepper.   Increase heat and add beer, allow to boil and reduce.  Stir through grrated cheese until melted.  Remove pan from heat and mix in egg yolks.  Set mixture aside.
Heat a grill and toast bread on one side until golden.  Turn over and place on a tray.  Spoon filling over the bread and grill until bubbling and golden.
Serve at once.