Preheat your oven to 350°F. While it heats, prepare your ingredients: cut the rhubarb stalks into 1/2-inch thick slices, coarsely chop the walnuts, and measure out all dry ingredients. Having everything ready before assembly ensures smooth, efficient cooking and prevents ingredients from sitting too long after cutting.

In a bowl, combine the butter, 1/2 cup sugar, flour, coarsely chopped walnuts, and cinnamon. Use a fork or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with pea-sized pieces throughout. I like to use Kerrygold butter here because its higher fat content creates a richer, more indulgent crumble texture. Set the crumble mixture aside while you prepare the filling.

In a separate bowl, toss together the sliced rhubarb, blackberries, 1 cup sugar, cornstarch, and vanilla extract until the fruit is evenly coated and the cornstarch dissolves slightly from the fruit's natural moisture. This coating prevents a watery filling and helps thicken the juices as the pie bakes. Let it sit for 1-2 minutes to allow the flavors to start melding.

Place the pie crust in a 9-inch pie dish (if not already in one). Pour the fruit filling from Step 3 into the crust, spreading it evenly. Distribute the crumble topping from Step 2 evenly over the filling, gently pressing it down slightly so it adheres but stays loose enough for crispness. I like to leave some gaps in the topping so steam can escape during baking, which helps prevent a soggy crust. Bake for 50-60 minutes until the crumble is golden brown and the fruit filling is bubbling around the edges.

Remove the pie from the oven and let it cool on a wire rack for at least 20-30 minutes before slicing. This resting time allows the filling to set slightly, making the pie easier to cut and serve. The crust will also firm up as it cools, preventing it from becoming soggy. Serve warm or at room temperature, optionally with vanilla ice cream or whipped cream.