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, toss together the rhubarb, strawberries, blueberries, and raspberries. Add the granulated sugar, brown sugar, cornstarch, cinnamon, vanilla extract, and lemon juice. Gently fold until all the fruit is evenly coated. Set aside to macerate while preparing the crust.
03 - Gently press the unbaked pie crust into a 9-inch pie dish. Trim any overhanging dough to about 1 inch beyond the rim, then fold the edges under and crimp decoratively.
04 - Spoon the berry-rhubarb mixture into the prepared pie crust, spreading it out in an even layer. Drizzle any accumulated juices from the bowl over the top of the fruit.
05 - In a medium bowl, whisk together the flour, rolled oats, brown sugar, salt, and cinnamon. Add the cold butter cubes and use your fingers or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.
06 - Scatter the crumble topping evenly over the fruit filling, covering the surface completely. Use your hands to gently press the topping down so it adheres to the fruit.
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 topping is deep golden brown and the filling is bubbling through the edges. Check at the 30-minute mark — if the topping is browning too quickly, loosely tent the pie with aluminum foil for the remaining bake time.
08 - Transfer the pie to a wire cooling rack and let it rest for at least 2 hours before slicing. This resting period allows the fruit juices to thicken and set properly. Serve warm or at room temperature, optionally with a scoop of vanilla ice cream or a dollop of whipped cream.