Preheat oven at 350-degree Fahrenheit and prepare a 9-inch round pie pan with a parchment paper. Keep it aside. (Cut out a big piece of parchment paper and place the pan on it. With a pen trace its circumference on the paper and cut it round shape. This will help to set the parchment paper perfectly at the bottom of the pie pan).
In a large bowl, combine fresh blueberries, maple syrup, vanilla extract, cornstarch, and lemon zest. Mix it with a spoon and keep it aside.
In another mixing bowl, combine oat flour, salt, maple syrup and water.
Then, form a soft dough with your hands. Make sure it binds well. (Do not over knead at this point). Now, place the dough between two parchment papers on a flat surface. With the help of a rolling pin, roll the dough flat in a round shape.
Gently, remove the first layer of parchment paper. Then, gently, release the flattened dough from the other parchment paper and place the dough at the center of the prepared pie pan. (Or you may invert the parchment paper with the crust dough on the prepared pan and then gently peel it off). With the help of your fingers press and shape its edges according to the pie pan. Make sure it does not break so shape until it starts to crack or break. (It may not cover the pan edges completely which is fine).
Poke all over the crust dough randomly with a fork and place the pan in the middle rack of the oven. Bake it for 10 minutes. Keep the oven on and take it out. Place the pan aside on a working counter surface.
In another bowl, combine rolled oats, oat flour and maple syrup. Mix it with hands.
Now, stir the blueberry filling again and spoon them out on the baked crust leaving the liquid in the bowl. Make sure to only spoon out the blueberries and not the liquid. Discard the leftover liquid and spread the blueberries evenly all though the baked crust. Cover its top evenly with oat crumble.
You may spray its top with cooking spray or egg wash with a brush. (Oat milk + maple syrup stirred in a bowl).
Place the pan again in the middle rack of the oven and bake for 25 minutes. When done, turn it off and take it out. Place it on a counter and let it cool down to room temperature. It will firm up as it cools. Slice and enjoy!