Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Line a baking tray with baking paper. Wash the apple, do not peel it! Quarter the apple, remove seeds and cut into small pieces.
Mix with cereal. Whisk cold buttermilk and egg. Mix 250 g flour, sugar, baking powder, baking soda and salt in a large bowl.
Add cold butter to the flour mixture. Rub in coarse crumbs with your fingertips. Do this quickly! Stir in the buttermilk and egg mixture briefly. Then add the apple and muesli mixture and mix in.
Stir only until just enough to form a dough.
For 8 large scones, pour the dough onto a lightly floured work surface. The dough is a little dry, which is good. Only knead lightly and shape into a rectangular plate (approx. 10 x 25 cm) about 2 cm thick with your hands.
Cut crosswise into pieces about 6 cm wide and place on the tray.
Bake in a hot oven for about 15 minutes until the scones have slightly coloured. Take them out and let them cool down on a cake rack for at least 10 minutes.