Preheat the oven to 190C fan, 200C, gas mark 6, put a flat baking tray in the oven to heat up. To make your pastry, mix together flour, sugar and salt. Pour in the rapeseed oil and stir until evenly mixed through. Make a well in the centre and pour in half of the water, use your hands to work the water in. Drizzle in more water as required, so that the pastry holds together and shape into a ball. Wrap the pastry in clingfilm and rest of at least 30 minutes. However, this pastry recipe is very forgiving so can be used immediately if you're in a hurry. The pastry will also keep in the fridge for up to three days and in the freezer for up to three months.
For the filling, peel, core and thinly size the apples into a bowl of cold water. Wash the blackberries. Roll out the pastry on a sheet of floured baking parchment. Drain the apples thoroughly and place the apples and blackberries into the centre of the pastry. Fold up the sides of the pastry into pleats working all the way around. Sprinkle the sugar onto the exposed fruit in the centre. Slide the galette onto the preheated baking tray and bake for 25-30 minutes.