Roll out your pie crust to about 12 inches in diameter and transfer it to a 9-inch pie pan, letting the edges hang slightly over the rim. Place it in the freezer while you prepare the crumble topping. In a medium bowl, whisk together the oats, flour, 1 cup brown sugar, salt, cinnamon, and ginger until evenly combined. Drizzle the melted and cooled butter over the mixture and use a fork or your fingertips to work it in until the mixture resembles coarse breadcrumbs with pea-sized clumps. Transfer the crumble topping to the refrigerator to chill while you work on the fruit filling.

Combine the rhubarb, raspberries, 1/4 cup brown sugar, 1/4 cup granulated sugar, finely ground instant tapioca, lime zest, lime juice, cardamom, and vanilla extract in a large bowl. Gently fold everything together until the fruit is evenly coated with the sugar and tapioca mixture. Let this sit at room temperature for 15 minutes—this allows the tapioca to absorb some of the fruit's natural juices and helps prevent a soupy filling. I find this resting time is crucial; it prevents excess liquid from making the crust soggy.

Preheat your oven to 400°F and place a baking sheet or piece of foil on the lower rack to catch any drips. Remove the chilled crust from the freezer and pour the macerated fruit filling directly into it, spreading it evenly. Scatter the chilled crumble topping from Step 1 evenly over the fruit, breaking up any large clumps as you go—don't pack it down, as you want it to stay loose and crispy. Place the pie directly on the preheated baking sheet and bake for 45 to 55 minutes, until the crumble topping is golden brown and the fruit filling is bubbling slightly at the edges. I recommend checking around the 40-minute mark; if the crumble is browning too quickly, you can tent it loosely with foil for the remaining time.

Remove the pie from the oven and let it cool at room temperature for at least 4 hours before slicing. This cooling time allows the filling to set properly so it slices cleanly instead of running all over the plate. The pie can be served at room temperature or slightly warmed, and it's wonderful with a scoop of vanilla ice cream or a dollop of whipped cream on the side.