Drape the pie crust over a 9 inch pie pan, gently pressing it to form to the pan. Trim any excess dough and flute the edges in your desired pattern. Place the pie pan into the refrigerator to chill while you prepare the crumb topping. This can be done a day in advance.
In a medium bowl, whisk together the flour, sugar, cinnamon and salt. Mix in the butter using your fingers, a pastry cutter or a food processor, mixing until it looks like coarse crumbs. Add oats and and mix a little more, using your fingers to create chunks with the crumbs. Place in the refrigerator to chill while you prepare the fruit filling. This can also be done a day in advance.
In a large bowl, toss the persimmons, apples, sugar, flour and cinnamon. Arrange the fruit filling in the pie pan. I like to alternate layers of apple with persimmon, but this is not necessary. Dot with pieces of the softened butter. Spread topping evenly over fruit and pat down gently with your fingertips.
Bake the pie for 25 minutes, then lower the heat to 350°F. Continue baking for another 30-40 minutes, or until the topping is golden brown, and the filling is bubbling within. Remove from the oven and let it cool completely before slicing.
Notes
Crumb topping adapted from In a Shaker Kitchen: 100 Recipes From the Shaker Tradition by Norma MacMillan. Filling based on Cinnamon Apple Crumble Pie via Epicurious. Persimmon Apple Crumb Pie Tips:
The pie crust and crumb topping can be prepared a day in advance and stored in the refrigerator until you are ready to assemble and bake the pie. Since working with a cold crust and crumb topping are critical when baking a pie to prevent crust shrinkage, keep it refrigerated until you are just about ready to fill the pie and bake.
For the filling, I recommend that you don't prepare this in advance. If you allow the fruit to sit in the sugar mixture for an extended amount of time, it will release juices and make for a soppy filling.
Resist the urge to slice into your pie while it is still warm, especially if you would like pretty, clean slices! Let the pie cool completely before slicing.
Fuyu persimmons should be ripe but not too soft. Keep in mind that hachiya persimmons are quite different from fuyu; definitely stick with fuyu persimmons in this recipe, which can be eaten while firm.
While the skin of a persimmon is edible, I prefer to peel them in this pie, but you can save time and leave it on. As for the apples, when I use Honeycrisp apples, I slice them thin and leave the skin on.
If you find that you fruit is already quite sweet, I like to use less sugar in the filling - around 1/3 cup if the fruit is naturally sweet, or if I am craving a less sweet pie.