Place your baking sheet in the oven and preheat to 375 degrees
In a mixing bowl combine sliced fruit, brown sugar, lemon juice cornstarch and spices, mixing to combine. Set aside
On a lightly floured surface oll out pastry dough to roughly a 14-16 inch circle. Transfer to a sheet of parchment paper. Brush the centre with egg white, leaving about a 2 inch border all the way around.
Pile fruit into the centre of the dough to cover egg white, trying not to add too much of the liquid. Fold edges up over the fruit, folding each section over the previous on to create a "pleat" (see photo above) until all the dough is folded up. Brush dough with remaining egg white.
Crumble Topping
Combine all ingredients for the topping in a large mixing bowl and using clean hands, rub butter into the mixture until fully distributed and mixture is crumbly. Sprinkle a generous layer of crumble over top of the exposed fruit.
Carefully remove preheated baking sheet from the oven and transfer the galette with the parchment on to the sheet. Return to oven and bake for 50-60 minutes until dark golden. Use a thin metal spatula to gently lift and check for doneness on the bottom. Remove from oven and cool.
Serve warm or cold.