Rhubarb pie is not an entirely common pie. But it should be! This pie has a delicious filling made from rhubarb with an almost crumble-like topping made from oats.
Preheat oven to 400°F. Line a baking sheet with parchment and set aside.
Prepare the streusel: In a bowl, combine the flour, oats, sugar, and salt. Cut the cold butter into ¼-inch pieces and add to the streusel mixture. Cut in with pastry cutter or fingers until incorporated, and only pea-sized pieces remain. Place in the refrigerator while preparing the filling.
½ cup all-purpose flour, ½ cup old fashioned oats, ⅓ cup light brown sugar, 6 tbsp unsalted butter, ¼ tsp kosher salt
Make the filling: In a large bowl, add the chopped rhubarb, sugar, instant tapioca (or cornstarch), orange juice, and salt. Mix together to incorporate.
6 cups diced rhubarb (cut into ¼-inch pieces), 1 cup granulated sugar, 3 tbsp instant tapioca (or 2 tbsp cornstarch), ½ tsp kosher salt, 2 tbsp fresh orange juice
Fill the pie: Pour the rhubarb mixture into a prepared, chilled pie shell. The pie will seem full, but rhubarb bakes down a lot. Sprinkle the streusel on top of the filling.
1 prepared 9-inch pie crust
Bake: Place the pie on the prepared baking sheet and bake it in the preheated 400°F oven for 10 minutes. Then reduce the oven temperature to 375°F and bake until the pie is bubbling throughout and the crust is golden brown, about 70-80 minutes. If the crust begins to brown too much, cover with a piece of foil (or pie crust cover) until fully baked.
Cool: Remove from the oven and let cool completely before cutting (at least 4 hours). The pie will finish baking and set up once it is out of the oven.