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Heat oven to 220C/200C fan/gas 7. Mix the flour, mustard powder, ½ tsp salt and a grinding of black pepper in a large bowl. Rub in the butter until the mixture resembles fine crumbs. Stir in half of the cheese and the sage. Mix together the egg and buttermilk in a separate bowl.
Make a well in the centre of the flour mix and pour in all but ½ tbsp of the buttermilk mix. Working quickly, stir until the mixture forms a soft, spongy dough. Tip onto a lightly floured surface and knead briefly until smooth. Roll out to a 3cm-thick square. Cut into quarters, then half each quarter diagonally, so you have 8 triangles. Place the scones on a floured baking tray, brush with the remaining buttermilk, sprinkle over the remaining cheese and top each with a sage leaf. Bake for 12-14 mins until they are well risen, golden and sound hollow when tapped on the bottom. Eat while still warm, spread with butter.