150g/5oz good-quality black cherry jam
750g/1½lb fresh red cherries, stalks and stones removed
250g/9oz plain flour, plus extra for dusting
1 tbsp ground arrowroot, mixed to a paste with 2 tbsp water
1 tbsp caster sugar, plus 2 tsp for topping
175g/6oz fridge-cold unsalted butter, cut into cubes
1 large free-range egg, beaten