In a large mixing bowl, combine the chopped rhubarb, granulated sugar, cornstarch, ground cinnamon, and lemon juice. Stir until the rhubarb is well coated and let it sit for about 10 minutes to release some juices.
Roll out the pie crust and place it in a 9-inch pie dish. Trim any excess crust hanging over the edges.
Pour the rhubarb mixture into the pie crust, spreading it evenly. Dot the filling with small pieces of butter.
Fold the edges of the pie crust over the filling, creating a rustic look. Leave the center open to allow steam to escape.
Brush the beaten egg over the crust and sprinkle the tablespoon of sugar on top.
Bake in the preheated oven for 15 minutes. Reduce the temperature to 350°F and continue baking for an additional 30-35 minutes, or until the filling is bubbly and the crust is golden brown.
Allow the pie to cool for at least 30 minutes before serving to let the filling set.