Preheat the oven to 220C/425F/Gas 7. Lightly flour a baking tray.
Sift the flour, salt, baking powder and mustard into a bowl.
Rub in the butter until the mixture resembles fine breadcrumbs. (You can do this quickly in a food processor.)
Grate half of the cheese very finely, and roughly chop the other half. Stir into the flour mixture with the chopped chives.
Gradually add milk to make a soft but not sticky dough (you may not need all the milk).
Turn the dough onto a floured surface and knead lightly, then roll the dough out into a thickness of just less than 2.5cm/1in. Use a 6cm/2½in cutter to cut the scones, re-rolling the off-cuts as necessary. Handle the dough as little as possible to keep it light.
Put the scones onto the floured baking tray, allowing at least 2.5cm/1in between them and glaze with the egg and milk mixture.
Bake for 10 minutes or until risen and golden brown.
To serve, finely slice the russet apples and fan half a dozen slices on the plate with the cheese scone and a small pat of butter.