Prepare a flaky double crust (using flour, shortening, salt, cold water).
Toss sliced rhubarb and strawberries with sugar and a little flour (for thickening). Line pie plate with bottom crust. Fill with fruit mixture and sprinkle with cinnamon if desired. Dot with butter. Apply top crust, seal edges, cut vents. Bake at 425?F for 15 min, then 350?F ~30 min until crust is golden and filling is bubbling. Cool before slicing.