Wash the rhubarb and cut it into 1/2 inch pieces. Wash and hull the strawberries. Cut any large strawberries in half.
In a large mixing bowl, stir together the rhubarb, strawberries, coconut sugar, lemon juice, and vanilla extract. Add the tapioca flour and stir until everything is well mixed.
Pour into the unbaked pie crust.
In a food processor, combine the crumble topping ingredients. Cut the butter into chunks before adding it to the food processor. Pulse until large crumbles form.
Sprinkle the crumble topping evenly over the pie filling. Place the pie plate on a rimmed cookie sheet.
Bake for 20 minutes. Without opening the oven door, reduce the temperature to 350° F and bake for 40-45 more minutes.
If the crust or crumble topping begins to get too brown, cover it with aluminum foil and finish baking. The filling should be soft and bubbly when the pie is done.
Allow the pie to cool (about 4 hours in the fridge) before cutting and serving. Store leftovers in the fridge.
Notes
To get extra pie crust dough, you may need to make another batch of the pie crust. However, I have often rolled out the pie crust dough thin enough so that I have enough for the crust and extra for the topping.