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 quickly bring the ball into a round ball with your hands. You don’t really need to work the pastry as there’s no gluten to activate. The pastry should still be a little sticky.
Wrap the pastry ball in greaseproof paper and flatten it slightly.
Place in the fridge for 30 minutes to chill.
Pre-heat the oven to 180°C.
Dust the work surface with a gluten-free flour blend then roll the pastry out into a circle large enough to line a 20cm tart tin.
Once you have lined the pastry in the tin and neatened the edges with a knife, place greaseproof paper over the pastry, so it comes up over the sides, then fill the tin with baking beans.
Place 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.
Place your nettle leaves in a large saucepan of salted boiling water. Bring back to the boil then simmer for 2 minutes.
Drain the nettle leaves and douse in cold water to stop them cooking any further. When cool enough to handle, ball up the nettle leaves and squeeze out the excess water. Chop finely then set aside.
In a large bowl whisk together the eggs and egg yolks with the crème fraiche.
Add the chives, then the cheddar, then the nettle leaves. Season with plenty of salt and pepper or bacon salt if you have it.
Pour the filling into the tart shell and place back in the oven for 25-30 minutes.
Remove from the oven and leave to cool to room temperature before trimming the edges off the pastry and taking it out of the tin. Serve warm.
*Pastry recipe adapted from Flaky Pie Dough in Alanna Taylor-Tobin's Alternative BakerThis recipe was updated in 2017 to be gluten-free, so the resulting pastry will not be as light in colour as those in the photos.