The amount of oatmeal depends on the size, shape and design of the final pie. Of course, you can always mix more dough, as needed; don’t be too worried. The pie is intended to be ‘tough’ and ‘rough’, at least initially when baked, so the shape might well be that of a cake with nearly vertical sides. If done in the right proportions, the pie can be removed from the pan, and wrapped and placed in the fridge, without collapse. This is intended to be the case, the pie is ‘tough’, TOP; it is intended to be ‘cured’ for several days or a week before eaten, to soften the crust and the inner crusty materials into something like dumpling pieces.
On a flat rolling surface (perhaps with a cloth) spread the oatmeal out with an equal amount of flour on top. Mix the two ingredients and gradually add water. Mix with plenty of flour on your hands and tools to minimize sticking. When a consistency somewhat uniform emerges, roll it out, keeping the sides intact as much as possible. Fold over double from the side and roll out. Fold over double from the other side, roll out, etc. Finally, when it seems to have some body, roll it out to be about 3 mm thick. It doesn’t matter much but rolling tends to make it tougher. One thought is to simply place the unrolled dough in the tin; if you do, it won’t be consistent in thickness and will be hard and brittle around the top.
Paint the tin with olive oil to lessen sticking. Fit the sheet of dough to the tin to form a shell, covering the whole inside and sides uniformly. Work the dough at the curves or joints to get rid of any lumps and smooth/straighten the shell. The shell is improved by ‘fermenting’ i.e., covering in the fridge for a day or two. It gains more body and flavour. As an aside, for a ‘nutty’ top, take about ½ a cup of oatmeal and add several tablespoons of olive oil. Let it set for a day or so; it makes a ‘nutty’ topping for your pie.
Fill your shell with the fruit as you like: Examples include apples, plums, rhubarb and persimmon. Even out your fruit so that they are not all sour and you will not need to add honey or sugar. Some possibilities for sweetening include jam and conserves. You can avoid sugar altogether sweet fresh figs or fig jam, dried pitted prunes or dates will do. Soft, ripe and sweet heritage persimmons (soft and squishy, ripe, astringent variety) and apples add significant sweetness. Ask yourself the question: “would you like to eat the combination you have put into the pie?” Mix up rough and use the taste test to see if the combination is OK; with sweet apples, about 1/3 rhubarb may work. Peel and section your apples; add the pieces to fill the shell to about 2cm off the bottom. Add the rhubarb; if cooked or canned, it will smooth and fill in between the apples. Add spices or sweetening and plenty of oatmeal flakes to roughly cover the fruit.
Persimmons make beautiful ‘stars’, sectioned across their ‘axis’. The Fuyu variety are non-astringent, are crisp like apples, but difficult to ripen; when squishy ripe, they also make sweet, ‘soft stuff’ that can be scooped out of the skin to make a contiguous layer. With Fuyus you also may have to remove a seed; the astringent, heritage variety are seedless. Cut the calyx off the top, section and place them to start a new layer.
Again, liberally sprinkle oatmeal throughout and every 2 cm or so make a partly completed layer of oatmeal dough in special ‘lasagna’ strips placed horizontally with space between them. Add more apples to 1 or 2 cm, more rhubarb, more oatmeal, more soft stuff, and another criss-crossed layer of dough; perhaps placed perpendicular to the first layer, for strengthening. Repeat until you fill the shell.
Prepare a dough top to cover the pie by rolling the extra dough out variously until it is about the right shape. Some left-over dough may be placed in the pie variously to improve the strength and the flavour of the pie. Now wet the uppermost edge of the shell with water and the meeting surface of the dough top. Place the dough top on the pie (flipping the dough top using the rolling surface or cloth for support). Pinch the top and sides together to make a crinkled edge seal that is not too thick. This is a hard and tough part of the pie and may need some real sawing before eating. Put some pretty holes in the top to let out steam; extra juice should ooze out here when the pie is cooking. Paint at least the upper edges (perhaps also the top) with olive oil and place in an oven at 120-150°C for one hour. No need to preheat. Check the pie with a sharp knife to be sure it is done. If the knife shows any resistance inside the pie, continue the cooking, perhaps another 15 minutes.
When done, leave the oven closed and allow the TOP to cook and cool overnight. Hours later, when cool, remove the pie from the dish by placing an appropriate tray on the top and turning the lot upside down. Run a (plastic) knife around the TOP/dish to separate the pie from the tray and place the TOP inside a plastic bag. Store the bag in the fridge for at least 2 days before eating. With some luck, the pie will not be runny and, turned over, will look like and can be cut like a cake! A piece of cardboard covered with Aluminium makes a good tray for the TOP bottom.