6cupschopped rhubarb1/2 in. pieces, approx 2-2½ pounds
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.
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.