In a large mixing bowl whisk together the gluten-free flours and icing sugar.
Slice the butter very thinly directly from the fridge and add to the flour.
Rub the mixture between fingertips until roughly shorn. It’s okay that the butter is still in largish pieces.
Whisk the whole egg and the extra egg white together in a small bowl.
Make a well in the centre and pour in the whisked eggs.
Bring the dough together using your hands.
Turn the pastry out on to a floured work surface and knead for about two minutes until the dough comes together into a smooth ball which is no longer sticky.
Flatten the ball slightly to make a thick round disc. Wrap in baking parchment and place in the fridge for 1 hour (or freezer for 15 minutes) to chill before rolling out.
Pre-heat the oven to 190°C / 170°C fan / gas mark 5.
Cut two large pieces of baking parchment, 2½ inches wider than the size you need to roll the pastry out to. Sprinkle flour on the inside of the parchment and place the chilled pastry disc between the parchment sheets.
Using a long rolling pin roll out the pastry into a round until it measures about 13 inches.
Peel away the baking parchment and use the rolling pin to lift the pastry over the baking tin.
Fit the pastry into the baking tin, patching up any cracking with excess pastry. Roll the rolling pin across the top of the baking tin to remove the excess pastry. Then you can see which parts of the pastry case look a bit thin so you can do further patching if necessary.
Place the baking tin in the for fridge for 1 hour (or freezer for 10 minutes) to chill.
Remove and place a sheet of baking parchment over the pastry and fill with baking beans.
Place the baking tin on a large baking tray to help get it in and out of the oven.
Place the baking tray with the tin in the middle of the oven and bake for 15 minutes.
Remove from the oven and discard the parchment and baking beans.
Prick a few holes into the bottom of the crust and bake for a further 12 minutes.
Remove from oven, allow to rest in the tin for 15 minutes before carefully removing the pastry crust from baking tin and transferring to a wire rack to cool completely before filling with the mascarpone cream and fruit.