2Stir together the egg, white sugar, flour, salt and rhubarb in a mixing bowl.
3Place one of the pie shells in the bottom of a 9 inch pie plate. Pour pie filling into shell; dot with butter. Put top crust over filling and cut slits into crust.
4Bake in preheated oven for 30 to 45 minutes; or cook 7 minutes in a microwave and then about 10 minutes in the oven.