Preheat oven to 350F/175°C. Chill a large mixing bowl in the fridge.
Line the bottom of a 9-inch quiche pan with parchment paper. Use a little coconut oil to grease sides.
In a high speed blender, blend the oats on high until they form a smooth flour (about 30 seconds). Blend until flour is as fine as possible.
To your large mixing bowl, add oat flour, sea salt and arrowroot. Mix with a metal spoon to combine ingredients.
Remove hardened coconut oil from fridge and using hands, quickly rub it into flour mixture until it resembles fine breadcrumbs (we're doing this quickly so that the oil melts as little as possible).
Add water to the bowl and mix in well using a metal spoon. Use hands to quickly shape mixture into a ball of dough.
Place a sheet of parchment paper on a flat surface, and place dough on top. Place another sheet of parchment paper over it.
Using a rolling pin, roll dough out until it is about an inch wider than your pie dish on all sides and roughly 6mm thick.
Remove top layer of parchment paper and place quiche pan upside down on top of pastry. Gently but quickly flip over so pastry is on top of the pan! (Be careful during this part, as it can be crumbly. If pastry breaks, don't worry - it should be fixable in the next step!)
Remove remaining layer of parchment paper, and very gently press pastry into pie dish, ensuring it reaches the corners. If you get any cracks, use a piece of excess pastry from the edge to fill it.
Using a flat knife, cut the edge of the pastry around the rim of the dish so you have a nice clean crust. (You will likely end up with a little to spare - feel free to use this as you wish!)
Bake crust in oven for 20 minutes, until it feels solid and crust is starting to come away from the pan (but before it turns golden).
Remove and allow it to cool in its pie dish for 10 minutes.