Instructions

• Preheat your oven to 350°F. While it heats, prepare your ingredients: cut the rhubarb stalks into ½-inch thick slices, coarsely chop the walnuts, and measure out all dry ingredients.
• In a bowl, combine the butter, ½ 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.
• 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.
• Place the pie crust in a 9-inch pie dish. Pour the fruit filling into the crust, spreading it evenly. Distribute the crumble topping evenly over the filling, gently pressing it down slightly.
• 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.
• Serve warm or at room temperature, optionally with vanilla ice cream or whipped cream.