Preheat oven to 400°. In a large bowl, combine diced rhubarb, flour, and sugar.
Brush the bottom of pie shell with egg whites.
In a small bowl, whisk together egg yolks and sour cream until thick. This will the creamy custard.
Add the egg yolk and sour cream mixture to rhubarb mixture and stir well.
Pour the rhubarb and custard mixture into the pie shell.
In another large bowl, combine oats, brown sugar, ground cinnamon, and butter. This will become the crumble topping for your pie.
Spread the mixture evenly over the pie filling. Bake for 10 minutes. Reduce the oven temperature to 350° and bake 50 minutes more.
Allow time to cool, serve, and enjoy your Rhubarb Custard Pie!