01 - Preheat oven to 375°F. Position the oven rack in the lower third of the oven. Line a baking sheet with parchment paper and set aside.
02 - In a large mixing bowl, combine the chopped rhubarb, sliced strawberries, blueberries, and raspberries. Add the granulated sugar, brown sugar, cornstarch, cinnamon, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated and set aside.
03 - Unroll or press the pie crust into a 9-inch pie dish. Trim any overhanging dough to about 1 inch beyond the rim, then fold under and crimp the edges as desired.
04 - Spoon the berry-rhubarb mixture evenly into the prepared pie crust, distributing the fruit in an even layer. Pour any accumulated juices from the bowl over the fruit.
05 - In a medium bowl, whisk together the flour, rolled oats, brown sugar, salt, and cinnamon. Add the cold cubed butter and work it in with your fingertips or a pastry blender until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
06 - Sprinkle the crumble topping evenly over the fruit filling, covering the surface completely. Use your hands to gently press some of the mixture together to create larger clumps for texture.
07 - Place the pie on the prepared baking sheet and bake for 50 to 55 minutes, until the crumble topping is deep golden brown and the filling is bubbling through the edges. If the topping begins to brown too quickly after 30 minutes, tent the pie loosely with aluminum foil.
08 - Transfer the pie to a wire cooling rack and let it cool for at least 2 hours so the filling has time to set. Slice and serve warm or at room temperature, optionally with a scoop of vanilla ice cream or a dollop of whipped cream.