Preheat the oven to 450 degrees F (220 degrees C).
Press the pie crust into a 9 inch pie pan. Spread rhubarb in an even layer in the bottom of the crust. In a medium bowl, whisk together the egg, white sugar, sour cream and 1/3 cup of flour until smooth. Pour over the rhubarb.
In a small bowl, mix together 1/2 cup of flour and brown sugar. Stir in melted butter until the mixture is crumbly. Sprinkle over the top of the pie.
Bake for 15 minutes in the preheated oven, then reduce the heat to 350 degrees F (175 degrees C). Continue to bake for 40 minutes, or until the edges have puffed, and the topping is golden. The center may still be slightly jiggly. Cool completely before slicing and serving.