Sift flour into a bowl with salt. Rub in the butter to breadcrumb stage.
Add cheese and mustard powder and stir in. Gradually add egg and enough milk to make a soft dough - you may not need all the milk.
Roll out on a lightly floured worktop to 2cm/3/4in thick. Cut into rounds.
Place on a lightly floured baking tray. Brush tops of scones with beaten egg.
Bake in a pre-heated oven Gas 6 400F 200C for around 10 minutes (depending on the size of the scones) until risen, golden, and firm to the touch.
Cool slightly, halve, then fill with Cashel Blue cheese.