Use all cherries if you wish, but the tartness of the blueberries seems to amplify the flavour of the cherries.
The cornflour becomes invisible, but effectively thickens the juices. Serves 6.
For the pastry: plain flour 230g; butter 140g; icing sugar 50g; eggs 1 large yolk, plus another beaten to seal and glaze the pie.
For the filling: cherries 800g; blueberries 200g; cornflour 2 tbsp; lemon 1; caster sugar 100g (plus a little extra).
You will also need a wide-rimmed metal pie plate or tart tin measuring approximately 26cm in diameter (including rim).
Make the pastry: put the flour into the bowl of a food processor, add the butter cut into pieces and process until the ingredients resemble fine, fresh breadcrumbs.
Mix in the icing sugar and the egg yolk. Transfer the mixture to a bowl, then bring the dough together with your hands to form a smooth ball.
Wrap the dough in parchment or cling film and refrigerate for 20 minutes.
Stone the cherries, put them in a mixing bowl then add the blueberries and the cornflour. Finely grate the lemon, add it to the cherries, then cut the lemon in half and squeeze the juice. Sprinkle the juice over the fruit and add the sugar.
Tumble the fruit, cornflour, juice, zest and sugar together and set aside.
Place an empty baking sheet in the oven, then preheat to 200C/gas mark 6. Cut the pastry in half. Roll out one half to fit the base of the pie plate, then lower on to the pie plate, leaving any overhanging pastry in place.
Spoon the filling into the dish, leaving a bare rim of pastry around the edge. Brush the rim with a little beaten egg.
Roll out the remaining pastry and place it over the top of the tart, pressing firmly around the rim to seal. Trim the pastry.
Brush the surface with beaten egg, pierce a small hole in the middle to let out any steam, then sprinkle the pie lightly with caster sugar.
Bake for 25-30 minutes, on the heated baking sheet, until golden.