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 rhubarb, 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, then 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 and pouring any accumulated juices over the top.
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, pea-sized crumbs.
06 - Scatter the crumble topping evenly over the fruit filling, covering the surface completely to the edges.
07 - Place the pie on the prepared parchment-lined baking sheet and bake for 50 to 55 minutes, until the crumble is deep golden brown and the filling is bubbling through the edges. If the topping begins to brown too quickly after 30 minutes, tent loosely with aluminum foil.
08 - Allow the pie to cool for at least 2 hours at room temperature so the filling sets properly. Slice and serve warm or at room temperature, accompanied by vanilla ice cream or freshly whipped cream if desired.