In a bowl, whisk together flour, salt & baking powder, then rub in the butter, before stirring in the cheese.
Peel & core the apples, then grate them coarsely, before stirring them into the flour mixture, along with the mustard, half of the beaten egg & enough apple juice to bind the mixture together into a soft dough.
On a lightly floured surface, gently roll out (or press) the dough into a round about 3/4-inch thick, then using a 3-and-1/2-inch plain round cutter, stamp out 6 rounds, re-rolling the trimmings as necessary.
Transfer these rounds to a baking sheet, then brush the tops with the remaining beaten egg & bake for 15-18 minutes, or until risen & golden.
Place on a wire rack to cool, before serving warm, split in half & buttered.