1. In a bowl, mix sugar, cornstarch, and butter. Gently stir in berries.
2. Scrape berry mixture into unbaked bottom crust in pan. Cover with top crust and flute edges. Line a 12-inch pizza pan or a 10- by 15-inch baking pan with foil (to catch drips) and set pie on foil.
3. Bake on bottom rack of a 375° regular oven or 350° convection oven until filling bubbles in center (check through cutout or slits), 45 to 55 minutes in regular oven, about 1 1/2 hours in convection. If crust browns too quickly (check after 40 minutes), cover pie loosely with foil.
4. Let pie cool on a rack at least 3 hours. Cut into wedges and serve, or chill airtight up to 1 day.