To make the crust, sift the flour onto some baking paper and then add it to a food processor.
Add the sugar, and salt and pulse a couple of times to mix. Followed by half of the butter and pulse several times.
Then add the rest of the butter and pulse 6 to 8 times until the largest pieces of butter are about the size of large peas.
Sprinkle the mixture with about 2 tablespoons of the ice water (leaving the cubes behind) then pulse again.
Then add more ice water, a tablespoon at a time, pulsing once or twice after each addition until the dough just barely begins to hold together.
You know that the mixture is ready if when you pinch some together with your fingers and it holds together. Be careful with the amount of water you add, too much and the crust will be tough.
Empty the crumbly dough mixture from the food processor onto a clean, dry, floured surface.
Form a mound with the mixture and knead briefly (just a couple of turns). Form into a ball, cover with plastic wrap and chill for at least 1 hour (although it can be up to a few days or even frozen at this point).
Once chilled, let the dough sit at room temperature for 10 minutes before rolling.
Preheat the oven to 400F/200C
Roll out the crust on a lightly floured surface to about ¼ inch thick. Transfer it to a 9" pie pan. Trim the edge or crimp if desired. Prick the bottom with a fork.
Cover the crust with baking paper and fill with baking beads. Blind bake for 10 minutes. Remove the paper and baking beads and set the crust aside.
For the filling:
In a large bowl, combine the sugars, flour and orange zest. Add the eggs and whisk until smooth. Gently stir in rhubarb and strawberries. Add the filling to the pie crust.
For the topping:
Add all of the ingredients to a food processor and pulse until combined.
Squeeze to form lumps and crumble over the pie.
Bake for 45-50 minutes (covering with foil if it starts looking too brown). Cool on a wire rack for at least 30 minutes before serving.