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, cinnamon, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated and set aside.
03 - 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 uniformly for consistent baking.
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 fingers 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.
07 - Place the pie on a parchment-lined baking sheet to catch any drips. Bake for 50 to 55 minutes until the topping is deeply golden brown and the filling is bubbling through the crumble. If the topping begins to brown too quickly, loosely tent the pie with aluminum foil after the first 30 minutes.
08 - Allow the pie to cool for at least 2 hours before slicing so the filling has time to set properly. Serve warm or at room temperature, optionally accompanied by vanilla ice cream or fresh whipped cream.