In a large bowl stir together 3/4 to 1 cup sugar, cornstarch, and salt. Add strawberries and rhubarb. Gently toss to coat. (If using frozen fruit, let mixture stand 15 to 30 minutes or until fruit is partially thawed, but still icy.) Transfer filling to a 9-inch deep-dish pie plate. Dot with butter or margarine.
Prepare pastry. On lightly floured surface roll pastry into a 12-inch circle. Cut slits to allow steam to escape. Place pastry on filling. Seal and crimp to the rim of the pie plate as desired. Sprinkle with sugar. Place pie plate on a baking sheet. Cover edge of pie with foil to prevent overbrowning. Bake in a 375 degree F oven for 25 minutes for fresh fruit (50 minutes for frozen fruit). Remove foil. Bake for 25 to 30 minutes more for fresh fruit (about 30 minutes for frozen fruit) or until top is golden. Cool on a rack. Makes 8 servings.