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 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 and set aside.
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 as desired.
04 - Spoon the prepared fruit mixture evenly into the lined pie crust, distributing the berries and rhubarb in a uniform layer. Pour any accumulated juices from the bowl over the fruit.
05 - In a medium bowl, whisk together the flour, rolled oats, brown sugar, salt, and cinnamon. Add the cold cubed butter and use your fingers or a pastry blender to work it into the dry ingredients 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. Gently press down to help the topping adhere to the fruit beneath.
07 - Place the pie 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, loosely tent with aluminum foil.
08 - Remove the pie from the oven and allow it to cool for at least 2 hours on a wire rack so the filling has time to set properly. Slice and serve warm or at room temperature, optionally accompanied by vanilla ice cream or freshly whipped cream.