Pre heat oven to 190C. Place flour, salt and mustard into a large mixing bowl and stir briefly.
Cut cold butter into small cubes and dust with the flour mixture. Crumble the Cashel Blue® cheese into the mixing bowl and stir to combine.
Using a bread knife, cut the butter and flour into smaller pieces until it resembles clumps of wet sand - a few larger pieces are fine as they'll add to the flaky texture overall.
Mix in the chilled water until combined and the dough comes away from the edge of the bowl; be careful not to over handle the dough.
Tip on to a very lightly floured surface and flatten into a rectangle about 2.5cm high. Cut out with a round cutter (about 4-5cm wide for scones), reshaping as necessary.
Place scones onto a baking tray and baste top of scones with the beaten egg. Bake for 20-25 mins mins, until a golden brown crust has formed on top.
Allow to cool slightly on a rack before splitting open and serving with Tracklements Original Onion Jam.