To make the filling, combine all ingredients except the flour in a large saucepan and bring to a simmer over medium heat while stirring.  Reduce heat to low and stir occasionally until rhubarb has broken down and the mixture has slightly thickened - about 30 minutes.

Cool the mixture overnight.  Once cooled, sprinkle flour into mixture and stir well until dissolved.  The flour will help thicken the filling and cook as the pie bakes.

To make the pastry, cut the butter into the flour until the mixture just comes together, using ice water as needed to help the dough come together.   Roll dough out to 1/8th inch thickness.

Cut 6inch rounds (we used a bowl) and transfer to a parchment lined baking sheet.  Fill each pie with 1 heaping tablespoon of filling.   Brush edges with beaten egg and fold over to seal, crimping edges with a fork and slashing the tops to allow steam to escape.

Once assembled, brush the tops of the pies with egg and sprinkle with sugar and oats or slivered almonds (if using).   Bake in a 400F preheated oven for 15-20 minutes or until browned and pastry is cooked through.  Let cool before serving.