Make the dough: In a large bowl, stir the flour, cornmeal, sugar, lemon zest, and salt. Using a pastry blender or two knives, begin to cut the butter into the flour. While the butter is still in large pieces, add the shortening to the bowl and continue to cut the fat into the flour until most pieces are the size of large peas.
With a big fork, stir in the lemon water, 1 to 2 Tbs. at a time, until the mixture looks shaggy but is moist enough to hold together when pressed. With well-floured hands, gently gather and press the dough into two equal disks, handling it only enough to make the edges of the disks reasonably smooth. Wrap the dough in plastic and chill for at least 60 minutes, but preferably 2 to 4 hours, before rolling.
Roll the bottom crust: Roll one disk of the dough out on a lightly floured surface into a 13-inch circle about 1/8 inch thick. Gently transfer the dough to a 9-inch metal, glass, or ceramic pie plate (I like to fold the dough in half and unfold it into the pan). Don't stretch the dough as you line the pan, or it will spring back when baked. If necessary, trim the overhanging dough to 1 inch from the edge of the pan. Refrigerate until needed.
Make the filling: In a large bowl, whisk 2/3 cup of the sugar with the cornstarch, allspice, cinnamon, and salt. Add the blueberries and blackberries and toss gently until thoroughly combined. Set aside.
Fill and top the pie: Roll the second disk of dough out on a lightly floured surface into a 13-inch circle about 1/8 inch thick. Using a sharp knife or pizza cutter, cut the dough into 3/4-inchwide strips. Pour the fruit filling into the pastry-lined pie plate, being sure to include any dry ingredients remaining in the bowl. Lay five of the dough strips over the pie, parallel to each other and spaced evenly (use longer strips in the center of the pie and shorter strips near the edges).
Want to see the making of a lattice-top pie crust in action? Check out our video
Carefully fold back the second and fourth strips a little past the center of the pie and lay a long strip of dough across the center of the pie, perpendicular to the other strips. Unfold the second and fourth strips over the perpendicular strip.
Next, fold back the first, third, and fifth strips and lay a new strip across the pie, perpendicular to the folded strips. Unfold the three strips over the new strip.
Use this alternating technique to weave in three more strips (two go on the other side of the pie), completing the lattice top and evenly covering the pie. Trim the strips to overhang the pie by 3/4 inch.
Roll the overhanging bottom dough and the strips together into a cylinder that rests on the edge of the pie pan.
Crimp the edge. Cut the cold 1 Tbs. butter into small pieces and dot over the open areas of the lattice. Freeze the assembled pie for about 15 minutes to relax the dough.
Meanwhile, position a rack in the center of the oven and set a foil-lined heavy-duty rimmed baking sheet on the rack. Heat the oven to 425°F. Just before baking, brush the lattice top with the melted butter and sprinkle with the remaining 1 Tbs. sugar. Put the pie on the heated baking sheet and bake for 15 minutes. Reduce the oven temperature to 375°F and bake until the fruit is bubbling consistently at the center of the pie, 60 to 80 minutes more. This is important—if it isn’t bubbling near the center, it hasn’t thickened yet. If the crust starts to get too brown, cover it loosely with foil during the last few minutes of the baking time.
Let the pie cool to just warm before serving.
Make Ahead Tips
The dough can be made up to 2 days ahead and refrigerated, or up to 2 months ahead and frozen. And the baked and cooled pie can be stored at room temperature for up to 2 days.
nutrition information (per serving):
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Photo: Scott Phillips