¼cuparrowroot or cornstarch or 6 Tbsps. all-purpose flour
1Tbsp. unsalted butter
Pie pastry for two 9-inch crusts
Trim off any vestige of green leaf (rhubarb leaves are toxic) and brown bits from the rhubarb and cut the stalks into 1-inch lengths.
Mix the sugar, cinnamon or orange peel, and the arrowroot or other thickener in a large bowl and toss with the rhubarb pieces. (I prefer arrowroot, which does not mute the flavor. Instant tapioca is not suitable for thickening lattice-top pies or open tarts without a top crust).
Let the rhubarb rest for 15 minutes.
Place half the pie dough on a floured surface and roll outward from the center in all directions to be about 3 inches wider than your 9-inch pie pan. Keep a spatula handy and check that the bottom crust is not sticking. You can lift the crust with a spatula and strew a little flour under the crust if necessary.
Work quickly so the butter doesn’t soften; mend any cracks with a dab of cold water and by adding a smidgen of dough or by overlapping the edges together. Roll up the bottom crust onto your rolling pin and unroll it over the pie pan, pressing it into the bottom corners.
Trim off any extra dough at the edge, allowing a ¾-inch overhang. Turn under the overhang. Refrigerate for at least 30 minutes to let the dough relax before filling.
Roll out the remaining square of dough into a rectangle. With a pastry wheel or a knife, cut either four wider strips or eight strips that are about half an inch wide. Refrigerate these until you are ready to fill the pie.
Preheat the oven to 425 degrees F. Pour the pie filling into the bottom crust and drop dots of butter over the rhubarb. Place half the lattice strips parallel over the pie. Weave the remaining strips perpendicular or diagonal to the first set.
Trim any overhang, press the strips into the edge of the pie and flute the edge of the pie crust. Bake the pie on a baking sheet covered with parchment for 20 minutes. Then reduce the heat to 350 degrees and bake for another 25 to 35 minutes or until the crust is golden and thick juices bubble up.
Allow the pie to cool on a rack 3 hours to let the filling thicken completely (if you can stand to wait that long).