In a large bowl, whisk together flour, salt, and sugar. Add the cold cubed butter and work it into the flour using a pastry cutter or your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
Drizzle in ice water, 1 tablespoon at a time, mixing until the dough just comes together. Divide the dough into two discs, wrap in plastic, and refrigerate for at least 30 minutes.
Wash and trim rhubarb stalks, removing any leaves (which contain toxic compounds). Cut into ½-inch pieces and place in a large bowl.
In a separate bowl, mix sugar, cornstarch, salt, and orange zest. Pour this mixture over the rhubarb and toss gently until evenly coated. Let sit for 15 minutes to allow the rhubarb to release some juice.
Preheat your oven to 425°F (220°C). Roll out one disc of dough on a floured surface to about 12 inches in diameter. Transfer to a 9-inch pie plate, pressing gently into the bottom and sides.
Pour the rhubarb filling into the crust and dot with the 2 tablespoons of butter pieces.
For a traditional lattice top: Roll out the second disc of dough, cut into 1-inch strips, and weave into a lattice pattern over the filling. Trim excess dough and crimp the edges decoratively.For a crumb topping: Mix flour, brown sugar, and cinnamon in a bowl. Cut in butter until crumbly. Sprinkle evenly over the filling. Place the pie on a baking sheet (to catch any drips) and bake at 425°F for 15 minutes. Reduce temperature to 375°F (190°C) and continue baking for 35-40 minutes, or until the crust is golden and the filling is bubbling. Shield the edges with foil if browning too quickly.
Allow the pie to cool for at least 3 hours before slicing to let the filling set properly.