Preheat your oven to 400°F (200°C). Place the unbaked pie crust into a 9-inch pie dish and crimp the edges nicely.

In a large bowl, combine the chopped rhubarb, strawberries, granulated sugar, cornstarch, grated ginger, and orange zest. Toss well until the fruit is evenly coated.

Pour the fruit filling into the prepared pie crust, spreading it out into an even layer.

In a medium bowl, prepare the crumble topping by whisking together the flour, oats, brown sugar, cinnamon, and salt.

Add the cold, cubed butter to the dry crumble ingredients. Use your fingers or a pastry cutter to work the butter into the mixture until it resembles coarse crumbs.

Generously sprinkle the crumble topping over the fruit filling, covering it completely to the edges.

Place the pie on a baking sheet to catch any drips and bake for 20 minutes at 400°F (200°C).

Reduce the oven temperature to 375°F (190°C) and bake for an additional 30 to 35 minutes, or until the topping is golden brown and the fruit filling is bubbling thickly.

Remove the pie from the oven and transfer it to a wire rack. Let it cool completely for at least 3 hours to allow the filling to set firmly before slicing.