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 the fruit is evenly coated and set aside to macerate.
03 - Gently 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 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, pea-sized crumbs.
06 - Sprinkle the crumble topping evenly over the fruit filling, covering the surface completely from edge to edge.
07 - Place the pie on a parchment-lined baking sheet to catch drips. Bake for 50 to 55 minutes until the topping is deep golden brown and the filling is bubbling through the crumble. If the top begins to brown too quickly, tent loosely 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 with a scoop of vanilla ice cream or a dollop of whipped cream.