Here's a recipe for the best rhubarb pie with a lattice pie crust. It's perfect! The rhubarb pie filling is thickened with tapioca starch (or tapioca flour), with just the right amount to allow the juices to thicken and set without being gluey. This pie is made with an all-butter pie crust so it is flaky and tender. It rolls out easily and the pie dough is easy because it's made in the food processor. Perfect pie shouldn't be hard with this recipe!
Slice the rhubarb into ½-inch to 1-inch chunks. Place half of the chopped rhubarb into a 8x8 non-reactive glass baking (I used a Pyrex baking dish you can find on Amazon). Mix it with the granulated sugar, maple sugar, tapioca flour, and salt. Cover tightly with aluminum foil. Bake for 30 minutes or until the fruit is soft and the juices have run.
Remove from the oven and add baking soda. Mix well. Add the rest of the rhubarb and the vanilla, stir, and transfer to a bowl then place in freezer to chill.
Meanwhile, make the easy all-butter pie dough in the food processor
In the bowl of a food processor fitted with the metal blade, combine the flour, sugar, and salt. Add the cold butter and pulse to form a coarse crumble.Add the water and pulse it in, then let the food processor run just until the dough comes together. I bought my Cuisinart Food Processor on Amazon.
Divide the dough in two and transfer both pieces to your work surface. Shape one piece into a disk. Shape the other into a rectangle. Wrap both in plastic wrap and chill for 1 hour.
After 45 minutes of chilling. Retrieve the rectangle of dough. Roll it to a 10x15 rectangle on a floured surface. Transfer to a big cookie sheet and chill again.
Roll the disk to a 14" circle. Transfer to metal pie plate and fit it in place. Pour the chilled filling into the pie and smooth it so it's even. Place in the fridge.
Retrieve the big 10x15 rectangle of dough. Cut it into at least 12 one-inch strips. Take the pie out of the fridge and place 6 strips side by side vertically.
Now weave in the other 6 strips horizontally to form a lattice (check out the video). Trim and crimp the edges.
Chill the pie in the fridge for 45 minutes. Meanwhile place a large cookie sheet on the bottom rack of the oven. You will bake the pie on this sheet so make sure there's enough headspace for a pie!
When the pie has chilled, brush the surface of the lattice with milk and sprinkle with turbinado. Bake the pie on the bottom rack on the preheated cookie sheet for 30 minutes at 400ºF (200 °C). Lower the oven temperature to 350ºF (175 °C) and continue baking until the edges and top of the pie crust are golden brown. The filling should be bubbling and very hot. This takes at least 50–60 minutes more of baking.
Let the pie cool to room temperature, preferably overnight (even 24 hours) so that the pie sets properly before serving.