Crust: Mix the flour, sugar and salt together in the food processor. Add chunks of cream cheese and process until the texture is coarse and slightly lumpy. Add the chunks of frozen butter and pulse until coarse with some pea-size lumps. Remove top of food processor and sprinkle in 2 tablespoons cream. Pulse until mixed. Reach in and pinch dough. If it doesn’t come together, add a third tablespoon of cream. At this point, it should look pretty crumbly, but dough should hold together when pinched. Empty crumbly mixture onto a clean surface and gently shape it into a disk. Chill for 2 hours to relax the gluten in the dough. Remove from refrigerator, let sit for 15 minutes, roll into a 12 inch circle and fit into a standard 9 inch deep dish pie plate. Return dough-lined pie dish to the refrigerator and chill while you prepare filling.
Topping: Prepare this right after you make the crust. Mix first 5 ingredients in bowl of food processor. Add cut up butter and pulse until crumbly, then sprinkle in the vanilla extract. Cover and store in the refrigerator until ready to use.
Filling: Place the rhubarb in a large bowl (here's where an 8 cup measuring cup is handy). Add the sugar, tapioca flour, and zest and stir gently to coat the rhubarb. Transfer the filling to the chilled pie shell. Spread evenly with a slight mound in the center. Sprinkle/crumble the streusel topping evenly over the pie to cover completely. Press it lightly to adhere.
Put the pie on the center rack, then set a pan for catching drips on the rack below it. Bake the pie at 400 for 20 minutes. Tent loosely with foil. Lower the oven temperature to 375 degrees and continue baking another 40 to 50 minutes. Cool completely on a wire rack.
And here's the apple filling I used.
Mix sugars, cinnamon, salt and flour in a large bowl. Put apples on top, toss butter over apples and toss everything to combine, coating the apples with sugar mixture. Toss in the lemon juice. Bake at 400 for 20 minutes, lay a piece of foil loosely over pie to shield, then bake at 375 for about 40 minutes. Make sure to put a rimmed pan on the rack below the pie to catch drips!