To make crust, blend flour and salt in a bowl. Cut shortening into flour until coarse crumbs form. Add water, 1 T. at a time, until mixture holds together. Form into 2 balls, wrap in plastic wrap and chill while making filling.
In a saucepan over medium low heat, cook rhubarb and strawberries, covered, 10 minutes. Add lemon juice and stir.
Combine sugar and cornstarch and add to saucepan. Stir and cook, uncovered, 10 minutes, or until thick and bubbly. Cool in pan.
Roll out 1 portion of pie crust on a floured board and fill pie pan. Pour filling over crust. Roll out second portion of pie crust and top. Cut slits in top and crimp.
Brush top with a beaten egg and sprinkle with 1 Tablespoon sugar.
Bake 30 minutes. Reduce heat to 350 degrees and bake an additional 10-15 minutes, until filling is bubbly and top has lightly browned. Cool.