6cupschopped rhubarb1/2 in. pieces, approx 2-2½ pounds
½cupsugar
½cupbrown sugarpacked
½tspkosher salt
1tspvanilla
¼tspcardamom
½cuptapioca flour
Instructions
Preheat oven to 425°F with rack in the bottom third of the oven. If you have one, place baking stone on rack.
Line 9-inch pie plate with a single crust, crimp edges and refrigerate/freeze for at least 10 minutes.
For the topping
Combine all the dry ingredients in a large bowl. Toss the butter with the dry ingredients and mix/rub with a fork/pastry cutter/your fingers until crumbly. You should have big and small lumps. Refrigerate until needed.
For the filling
Toss the rhubarb, sugar, brown sugar, salt, vanilla, and cardamom in a large bowl. Add the tapioca flour and mix until no dry spots of flour remain. Let sit for 20 minutes.
Assemble:
Pour filling into the crust. Scatter the topping evenly over the top. Place pie plate on foil-lined rimmed baking sheet in the oven.
Bake for 10 minutes at 425°F. Reduce oven to 375°F and bake for an additional 40-50 minutes, until the crust is a deep golden brown and filling is bubbling. You may need to cover the edges/top if it starts to brown too much, so check after 30-40 minutes.
Try to let the pie cool until close to room temperature before serving to give the pie a chance to set.