Mixed Berry Pie (Made with Frozen Berries)

"[Photograph: Lauren Weisenthal] Frozen berries are a great alternative to fresh for winter baking. Many people fret about whether they should thaw the fruit first, but you don't need to. With the exception of a slightly longer baking time, the results are no different from baking with fresh berries, and the crust actually holds its shape well with frozen berries inside. Feel free to adjust the sugar and cornstarch to your tastes for sweetness and texture; less cornstarch will yield a runny pie, more will give the filling a more jammy texture...."