Peel and core the apples, then dice the apples into cubes. Add them to a large bowl.
Slice and dice the rhubarb into cubes as well. Small pieces are important so that the rhubarb will soften during baking. Add the rhubarb to the bowl, along.
Gently wash the blueberries and blot them dry with a clean dish towel. They don't have to be bone-dry, just remove the excess water. Add them to the bowl.
Hull the strawberries, then slice them into thin slices. Add them to that bowl!
Gently wash the raspberries and blackberries, and blot them dry with a clean dish towel. Add them to the bowl. (If the blackberries are really big, cut them into a few smaller pieces.)
Sprinkle the granulated sugar over the fruit and gently stir the sugar and berries together.
Blend the syrup and thickeners
Pour the maple syrup and vanilla extract into a small bowl and stir them together.
Sprinkle the cornstarch, tapioca flour, and salt into the bowl and whisk everything well. Whisk until there are no lumps or dry specks remaining.
Pour the syrup mixture over the fruit, and gently stir so all of the fruit is coated.
Allow the fruit mixture to rest, covered, for about 30 minutes. This will give the thickeners time to start working.
Prepare the pie dough
While the fruit rests, get the bottom crust of the pie ready. Have ready a 9-inch pie dish.
Dust a work surface lightly with flour. Roll out half of the chilled pie dough into a 12-inch circle.
Drape it over your rolling pin to move the dough into the pie dish. Press the dough into the pie dish bottom and along the sides. Leave a 1-inch overhang all around, trimming excess away. (You can save the trimmed bits to make decorative cut-outs for the top.) Press a sheet of plastic wrap over the surface of the dough and place the pan in the fridge.
Take out the other half of the dough. Roll this on a lightly floured surface into a 12-inch circle. This is going into the fridge too, so dust a cutting board or large plate with flour and transfer the dough circle to it. Cover it with a plastic wrap and pop it into the fridge.
Assemble the pie
Preheat the oven to 400° F. If you're using a metal pie dish leave the oven rack in the center of the oven. For glass or ceramic dishes, move the oven rack down one rung closer to the bottom of the oven. Have ready a baking sheet large enough to hold your pie dish.
Whisk the egg together with the tablespoon of water in a small dish. Hold this aside.
Once the fruit has thickened for 30 minutes, give it a stir to distribute the juices and fruit evenly.
Take out the dough-lined pie dish. Pour the fruit and juices into the pie shell and spread the filling out.
Take out the other circle of dough. Drape it over your rolling pin to move it over the pie, and lay the dough over the top.
Trim the edge to be just a little longer than the bottom edge. Roll the top edge underneath the bottom edge all around. (If it's too thick in a spot, trim it back a little.) Go around again crimping the edge firmly down. Refrigerate the pie for 10 minutes.
Roll out the dough scraps and use a knife or cookie cutters to cut out a few decorative shapes for the top.
Take your pie out of the fridge. Add your dough cut-outs to the top along with a couple of decorative slashes so steam can vent during baking. Brush the top crust with the egg wash, then sprinkle a little decorative sugar over the crust.
Place the pie on the baking sheet and put it in the oven. (The baking sheet will catch any melting butter from the crust and fruit juice from the filling.) Bake the pie for 30 minutes.
The top should be nicely browned by this point. Drape a sheet of aluminum foil over the top and bake the pie for 5 minutes more - this is to give the filling more time to set, while keeping the crust from over-browning.
Move the pie to a cooling rack. Here's the hard part: let the pie cool COMPLETELY! This is another step to let the juicy fruit filling set. If you cut into the pie while it's hot, the filling will run out all over the place. Give it a couple of hours, and point a fan at the pie to help it along.
Once the pie is cool, cut out slices and serve. They're really tasty with a dollop of sweetened whipped cream! Makes about 8 slices. Store leftover pie covered in the fridge.
The berry amounts are listed in ounces as well as in cups. If you're buying berries by the pint, keep in mind that one pint is roughly 11-12 ounces.
When not in season nor available from local growers, most of these fruits are available year-round in grocery stores. Rhubarb may perhaps be a little harder to track down, but you may be able to find it frozen. If you're growing rhubarb, it's a great idea to freeze some stalks to have on hand later!
Substituting frozen fruit: frozen fruits, when cooked, release a lot of water, and this can mean a runnier pie filling. Raspberries and strawberries are especially prone to this. Frozen blueberries and blackberries should be alright to substitute, but try to stick to fresh strawberries and raspberries. Since fresh rhubarb has a limited season, frozen can be used if you let it thaw and drain away the excess moisture before chopping it.
I like using Vermont Grade B maple syrup in this pie: grade B is darker with a stronger flavor. However any grade of pure maple syrup will work in this pie - just make sure it's pure maple and not that awful maple-flavored corn syrup.
Store-bought pie crust dough can be substituted, but the flavor and flakiness of the All-Butter Pie Crust Dough will be superior, and a delicious accompaniment to the fruit. Give the All-Butter Pie Crust Dough a try - it can be made ahead of time and stored for 3 days in the fridge or longer in the freezer.