Start by making the pastry. Place the flour, cornflour and caster sugar together into a bowl. Mix well. Add the butter and rub well to form a fine breadcrumb-like consistency. Add the yoghurt, lemon juice, salt and one beaten egg and mix to form smooth dough. Bring the pastry together into a ball, flatten wrap well and chill in the fridge.
Meanwhile, prepare the fruit. It can be sliced as thickly or thinly as you wish, where appropriate. Sprinkle the sugar over the fruit and leave it to macerate.
Once the pastry has been chilled for at least 20 minutes, preheat the oven to 180C/Gas Mark 4 and roll out to form a circle around 40cm in diameter. Transfer to a large non-stick baking sheet. Pile the fruit in the centre of the pastry and cold it up, leaving the centre uncovered by pastry. Brush the remaining beaten egg over the pastry. Sprinkle the demerara sugar over the pastry. Bake for 35-45 minutes until the fruit is tender and pastry is golden brown. Serve warm or cold, with cream, ice cream or custard.