Preheat oven to 450 degrees. Place rack on lowest rack in oven.
Combine sugar and flour in bowl. Sprinkle 1/4 of the mixture over pastry in pie plate.
Pile chopped rhubarb over this mixture. Add cinnamon to remaining flour and sugar mixture. Sprinkle rhubarb with the flour sugar mixture. Dot with small pieces of butter.
Cover with top crust. Brush with beaten egg, and sprinkle with sugar, if desired. Make sure to cut slits in the top of the crust to allow steam to escape.
Reduce oven temperature to 350 degrees F (175 degrees C), and continue baking for 40-45 minutes. For best results, let pie sit for a couple of hours to allow filling to set up!
-First tip when making a home made pie. Use home made crust.
-Do not mix the rhubarb with the sugar before placing in pan. The sugar layer on bottom is essential to this recipe working. Rhubarb gives off a lot of water, so the flour/sugar mixture will thicken the juice.
-The butter in the pie adds flavor and helps the filing to solidify.