Preheat the oven to 425°F
Divide the pastry more or less in half, with one piece slightly bigger than the other. On a lightly floured surface, roll out the larger piece to a circle about an inch bigger than your pie plate; drape it over the rolling pin and transfer to the pie plate, pressing it gently to fit, letting the sides hang over. Roll the other piece out to about the size of the top of the pie.
In a small bowl, stir together the sugar and flour. Put the berries into a medium bowl and add the sugar mixture and lemon zest; gently stir to combine, then pour into the pastry shell. Top with bits of butter.
If you like, cut the second piece of pastry into strips and make a lattice top; otherwise, lay it over the pie, brushing the edge of the bottom crust with a little beaten egg first, if you like, and trim and crimp the edge. Cut a few slits in the top to let steam escape.
Bake for 15 minutes, then turn the heat down to 350°F and bake for another 50-60 minutes, until golden and bubbling. Let cool before slicing.