To prepare the pie crust: Prepare the shortcrust pastry as instructed. Roll the pastry out into a large circle, then fit into a lightly greased 9-inch (23-cm) tart pan with a removable bottom, pressing it down gently so it eases into the bottom and sides of the plate. Trim the excess dough. (You can also use a regular pie plate and crimp the edges, as desired.) Refrigerate for 30 minutes.
Preheat the oven to 350°F (175°C). Prick the bottom of the pastry all over with a fork, then cover with a piece of parchment paper and add pie weights (or dry peas). Bake the crust for 15 minutes, then remove the pie weights and parchment paper and cook for 5 minutes more. Remove from the oven and reserve.
Increase the oven temperature to 400°F (200°C). Line a baking sheet with parchment paper. (You will set the pie dish on this baking sheet to catch overflowing juices and prevent a mess.)
To prepare the filling: Whisk the sugar and cornstarch together in a large bowl. Remove 2 tbsp (30 ml) of the sugar mixture and set aside. Add the orange zest and salt to the large bowl, whisk, then add the rhubarb and strawberries and mix to coat the fruits with sugar. Let sit while you prepare the crumble topping.
To make the crumble topping: In a large bowl, whisk the oats, flour, sugars, salt, and spice together. Pour the oil over the dry ingredients, then use a fork to distribute and mix it in until the mixture comes together in large crumbs. Mix in the sliced almonds.
To assemble the pie: Sprinkle the 2 tbsp (30 ml) sugar/cornstarch mixture over the bottom of the blind-baked crust. (This will soak up excess fruit juices and prevent the bottom of the pie from becoming soggy.) Pour the fruit mixture into the crust, spreading it evenly. Cover the pie the crumble mixture, pressing it down gently to even out the top of the pie.
Place the pie on the parchment paper-lined baking sheet and bake for about 45 minutes, or until the juices are thick and bubbly. Check on the crumble topping at around the 35-minute mark: if it looks pretty and golden brown, you may want to set a sheet of aluminum foil over it to prevent it from becoming too dark and the almonds to burn.
Remove the strawberry rhubarb crumble pie from the oven and transfer to a wire rack. Allow the pie to cool and rest for several hours (a half-day is best). This will allow the filling to set and make the pie easier to serve.