In a large bowl, mix sugar, flour, and salt. Stir in rhubarb and strawberries until evenly coated.
Roll out the pie crust and fit one into a 9-inch pie plate.
Pour the rhubarb and strawberry mixture into the crust. Dot with butter and sprinkle with cinnamon, if using.
Cover with top crust, seal edges, and cut slits to allow steam to escape.
Bake in the preheated oven for 15 minutes. Reduce the temperature to 350°F (175°C) and continue baking for 45 minutes or until filling is bubbly and crust is golden brown.
Cool on a wire rack before serving.
Notes
For best results, use fresh ingredients. You can also use a lattice crust for a traditional look.