Combine sugar, flour, and salt; mix well. Set aside.
Combine rhubarb and strawberries; toss lightly to mix.
Place half of fruit mixture in the bottom of a greased 2 1/2- quart baking dish. Sprinkle half of flour mixture over fruit; repeat procedure with remaining fruit and flour mixture. Sprinkle lemon juice evenly over top, and dot with butter.
Roll pastry to 1/4-inch thickness on a lightly floured surface. Place pastry over fruit mixture, sealing edges to sides of dish. Flute edges of pastry. Cut slits in top crust to allow steam to escape. Bake at 450° for 15 minutes. Reduce heat to 375°, and bake an additional 30 minutes or until crust is golden brown.