Preheat your oven to 350F/175°C. Line bottom of an 8-inch pie or flan dish with parchment paper and grease sides with coconut oil.
In a blender, blend oats on high power until they form a smooth flour (about 30 seconds). Keep blending until flour is as fine as possible. Pour into large mixing bowl.
Add almonds to blender and blitz on high power until they too form a smooth flour (you won't be able to get it as fine as the oats, but aim for as smooth as possible). Pour into large mixing bowl.
Add cacao, sea salt and arrowroot to bowl. Mix with a metal spoon to combine ingredients.
Using your hands, quickly rub coconut oil into flour mixture until it resembles fine breadcrumbs (we're doing this quickly so oil melts as little as possible).
Add maple syrup and water to bowl and mix using a metal spoon to combine.
Use your hands to quickly shape mixture into a ball of dough.
Here's the fiddly bit! Place a sheet of parchment paper on a flat surface, and place dough on top. Place another sheet of parchment paper on top of dough.
Using a rolling pin, roll dough out until it is about an inch wider than circumference of pie/flan dish and roughly 6mm thick.
Remove top layer of parchment paper from pastry and place pie dish upside down on top of it. Quickly flip it over so pastry is on top of dish! (Be careful during this part, pastry can be crumbly. If it breaks, don't worry - it should be fixable in next step!)
Carefully remove remaining layer of parchment paper, and very gently push pastry into pie dish, pressing it into corners of dish. If you get any cracks at this point, simply use a piece of excess pastry to plug them.
Using a flat knife cut edge of pastry around rim of dish or just below, so you have a clean crust. (You will likely end up with a little spare pastry - feel free to use this as you wish! I like making cookies from it.)
Bake in the oven 15-20 minutes, until crust is just starting to firm up.
Remove and allow to 'rest' in pie dish for 10 minutes, then carefully turn out onto a wire rack to cool completely. (Do not remove crust from dish before this! It will be soft and likely break.)