For the pastry, whisk the flour, salt and sugar together in a large bowl. If the butter is very hard, leave it at room temperature for a few minutes to soften slightly.
Rub the butter into the flour with your fingertips until it starts to clump together. Add the sour cream and mix to form a dough. If it's too wet, add a little more flour. If it's too dry, add a splash more sour cream.
Gather the dough together without overworking it. Flatten it into a disc about 2 cm thick, wrap it tightly and chill for at least 2 hours or overnight.
When you're ready to bake, preheat the oven to 200°C (400°F) and line a baking tray with baking paper.
For the filling, stir the cherries, sugar, cornflour, salt, lemon zest, lemon juice and vanilla together in a bowl until the fruit is evenly coated.
Roll the pastry out on a lightly floured surface into a circle about 30 cm (12 inch) wide. Roll it loosely over your rolling pin and transfer it to the lined tray.
Pile the filling into the centre, leaving a wide border. Fold the edges up over the fruit, overlapping the pastry as you go. Brush the pastry with beaten egg and sprinkle with sugar.
Bake for 40 to 45 minutes until the pastry is golden and the fruit is bubbling. After about 10 minutes, cover the galette loosely with foil to stop it browning too much.
Serve warm or at room temperature with vanilla ice cream.