01 - Preheat oven to 375°F. Position the oven rack in the lower third of the oven for even bottom crust baking.
02 - In a large mixing bowl, combine the chopped rhubarb, sliced strawberries, blueberries, and raspberries. Add the granulated sugar, brown sugar, cornstarch, ground cinnamon, vanilla extract, and lemon juice. Toss gently until the fruit is evenly coated and set aside to macerate.
03 - Gently press the unbaked pie crust into a 9-inch pie dish. Trim any excess dough hanging over the edges and crimp the border decoratively 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 all-purpose flour, rolled oats, light 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, pea-sized crumbs.
06 - Scatter the crumble topping evenly over the fruit filling, covering the surface completely. Avoid pressing the crumble down to maintain a light, crunchy texture.
07 - Place the pie on a parchment-lined baking sheet to catch any drips. Bake at 375°F for 50 to 55 minutes, until the crumble is deep golden brown and the filling is bubbling through the topping. If the crumble begins to darken too quickly, loosely tent the pie with aluminum foil after the first 30 minutes.
08 - Allow the pie to cool at room temperature for at least 2 hours so the filling sets properly before slicing. Serve warm or at room temperature, optionally accompanied by vanilla ice cream or freshly whipped cream.