1 tablespoon cornstarch
Juice of 1 lemon
6 cups blackberries
1/3 to 1/2 cup sugar
1 tablespoon lemon zest
1 2/3 cups all-purpose flour, plus more for dusting
1/2 cup blanched almonds
1/3 cup granulated sugar
1/4 teaspoon fine salt
12 tablespoons (1 1/2 sticks) cold unsalted butter, cut into small dice
3 to 4 tablespoons ice water
Vanilla ice cream, for serving, optional
Adjust the oven rack to the lowest position; preheat the oven to 375 degrees F. Coat the bottom and sides of a 9-by-13-inch baking pan with cooking spray.
For the filling: Dissolve the cornstarch in 2 tablespoons water and the lemon juice. In a large bowl, combine the blackberries, sugar and lemon zest (Depending on the sweetness of the blackberries, start with 1/3 cup sugar and add more as needed.) Pour the cornstarch mixture over the berries; toss to coat. Pour the filling into the prepared baking pan.
For the crust: Combine the flour, almonds, sugar, salt and butter in a food processor. Process until the mixture resembles a coarse meal. Add the ice water, 1 tablespoon at a time, pulsing until the dough begins to pull from the sides of the bowl. Gather the dough together on a floured work surface and pat it into a rectangle.
Roll out the dough into a 1/4-inch-thick rectangle that measures about 11-by-15 inches. Transfer the crust to the baking pan, laying it directly on the blackberry filling. (Any overhanging crust can be rolled inward.) Prick the crust a couple times with a fork.
Bake until the crust is golden brown and the filling is thick, 35 to 45 minutes. If the top starts to get too dark, cover it with aluminum foil and continue cooking.
Spoon the cobbler onto plates and serve with vanilla ice cream if desired.