Preheat the oven to 200ºC/400ºF/gas 6. Line two trays with greaseproof paper and rub with olive oil. Whiz the apple in a food processor until finely chopped, then add the flour, cubed butter, sugar and a pinch of sea salt. Blitz to fine crumbs for 1 minute, then remove 3 tablespoons of the mix and set aside. Pulse in the egg until combined, stopping to scrape down the sides, if needed.
Divide into 24 pieces, roll into balls, then press down lightly into 4cm rounds, lining them up on the trays as you go. Sprinkle over the reserved mix, lightly pressing it into the cookies. Bake for 8 to 10 minutes, or until lightly golden. Leave to cool slightly, then transfer to a wire cooling rack. Yum!