In a large mixing bowl combine the flours, chia seeds and salt.
Rub the butter into the flour in between your fingertips so it resembles very rough breadcrumbs then stir in the beaten egg with a fork.
Add the water 1 tablespoon at a time and start to bring the dough together with a pastry scraper. It should start to form quite quickly.
Tip the dough onto the work surface and bring the ball into a round ball. You want the pastry to still be a little sticky.
Wrap the pastry in baking parchment and flatten the ball slightly.
Place in the fridge for 30 minutes to chill.
Pre-heat the oven to 180°C / 160°C fan / gas mark 4
Dust the work surface with a gluten-free flour blend then roll the pastry out into a circle large enough to line a 25cm tart tin.
Once you have lined the pastry in the tin and neatened the edges with a knife, place baking parchment over the pastry, so it comes up the sides, then fill the tin with baking beans.
Place the tart tin in the oven for 20 minutes. Take out of the oven then remove the baking beans and parchment and brush the surface of the pastry with the beaten egg.
Place back in the oven for a final five minutes to seal the pastry. Remove from the oven and leave to cool to room temperature before adding the filling.