Preheat your oven to 400°F (200°C). Fit an unbaked pie crust into a 9-inch pie plate and crimp edges.

In a mixing bowl, whisk together granulated sugar, 1 tablespoon of flour, sour cream, egg, vanilla extract, and salt until smooth. Fold in chopped rhubarb.

Pour the filling into the prepared crust and smooth the top with a spatula.

Mix the remaining flour and brown sugar in a bowl. Cut in cold butter until crumbly, then sprinkle over the filling.

Bake the pie at 400°F for 15 minutes, then reduce to 350°F (175°C) and bake for 30-35 minutes until golden brown.

Allow the pie to cool completely at room temperature before refrigerating for a few hours before serving.