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, light brown sugar, cornstarch, ground cinnamon, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated and set aside to macerate.
03 - Lay the unbaked pie crust into a 9-inch pie dish, pressing it gently into the bottom and sides. Trim any overhanging dough to about 1 inch beyond the rim, then fold the edges under and crimp decoratively as desired.
04 - Spoon the berry-rhubarb mixture evenly into the prepared pie crust, distributing the fruit in an even layer and allowing the juices to settle naturally.
05 - In a medium bowl, whisk together the all-purpose flour, rolled oats, light brown sugar, salt, and ground cinnamon. Add the cold cubed butter and use your fingers or a pastry blender to cut it in until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
06 - Scatter the crumble mixture evenly and generously over the entire surface of the fruit filling, ensuring full coverage for a uniform golden crust.
07 - Place the pie dish on a parchment-lined baking sheet to catch any drips. Bake for 50 to 55 minutes until the crumble topping is deep golden brown and the filling is bubbling through the edges. If the top begins to brown too quickly after the first 30 minutes, tent it loosely with aluminum foil.
08 - Remove from the oven and 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, optionally accompanied by vanilla ice cream or fresh whipped cream.