To make the crust, combine the flours, sugar, and salt in a mixing bowl. Using a pastry blender or two forks, cut in the butter until it resembles coarse meal. It’s okay if the butter bits are different sizes, so long as none of them are larger than peas. At this point, add the ice water a tablespoon at a time and begin to gather the dough together. Turn the dough over itself a few times, but be careful not to overwork it. Pat the dough into a round disc and wrap in plastic to chill for a couple of hours (or at least 30 minutes).
To make the crumble, melt the butter over medium heat in a small saucepan. It will start to bubble and foam. Soon you will see little brown specks at the bottom of the saucepan, and smell a wonderfully nutty fragrance of freshly browned butter. Take the saucepan off the heat and let it cool.
Meanwhile, in a small bowl combine the oats, walnuts, brown sugar, flours, cinnamon, and salt. Once the butter has cooled somewhat but is still fluid, add it to the oat mixture and mix until incorporated. Set aside, or refrigerate for later.
To make the pie: Flour your work surface and place the unwrapped pie dough in the center. Using your favorite rolling pin (I prefer a French tapered pin, like this), press along the edges of the round, broadening the circle. You can move the disc around with your hands as you do this, making sure to flour the surface again when needed. Begin to flatten the pie dough into a larger circle by rolling from the center out. Roll, then rotate the disc and roll again. Don’t forget to keep flouring the surface. You can flip the disc and repeat this process until you have a circle of even thickness, about 12 inches in diameter. Invert your pie tin or dish onto the circle, and use a pastry cutter or knife to trim the dough, leaving a 1-inch border around the tin. Remove the pie tin and fold the dough in half. Place the folded dough into the pie tin, unfold it, and gently press it in, making sure it’s centered and fitted properly. To create a crimped edge, roll up the dough overhang toward the center of the pie, creating a ring of dough. Use the thumb and index finger of one hand to make a “V” and use the index finger of your other hand to press into the “V,” making a crimp. Continue until the entire ring of dough is crimped. At this point, you can put the crust in the refrigerator while you make the rhubarb filling. Congratulations, by the way! The hardest part is over.
Preheat your oven to 425°F/218°C with a rack on the lowest level.
Mix the rhubarb, granulated sugar, instant tapioca, orange zest, cinnamon, and salt in a bowl until uniformly combined.
Remove the rolled-out pie crust from the refrigerator and use an off-set spatula or spoon to spread an even layer of cream cheese on the bottom of the crust. Fill the shell with the rhubarb mixture, and top with the walnut crumble.
Bake the pie on a sheet tray (lined with aluminum foil for easier clean-up if you wish) at 425°F/218°C for 15 minutes, then lower the temperature to 375°F/190˚C and bake until the juices are bubbling all over and the crumble is browned, another 40 to 50 minutes. Cool the pie for at least 2 hours before slicing.