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 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 - 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 butter cubes and use your fingers or a pastry blender to work 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.
07 - Place the pie on a parchment-lined baking sheet to catch any drips. Bake for 50 to 55 minutes until the topping is deep golden brown and the filling is visibly bubbling around the edges. If the crumble begins to darken too quickly, loosely tent the top 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. Serve warm or at room temperature, optionally accompanied by vanilla ice cream or fresh whipped cream.