In a large bowl, whisk together the flour, almond flour, sugar substitute, and salt.

Add the cold cubed butter and cut it in with your fingertips or a pastry cutter until the mixture looks like coarse crumbs with some pea-sized pieces of butter left.

Sprinkle in the ice water, 1 tbsp at a time, mixing gently until the dough just comes together when pressed. Don't overwork it or the crust will turn tough instead of flaky.

Shape the dough into a disk, wrap it in plastic, and chill for 30 minutes.

While the dough chills, toss the blackberries with the sugar substitute, almond flour, cornstarch, lemon juice, lemon zest, and almond extract in a mixing bowl. Let sit for 10 minutes so the berries release some juice.

Heat the oven to 200 C / 400 F and line a 13 x 18-inch sheet pan with parchment paper.

Roll the chilled dough between two sheets of parchment into a rough 30 cm / 12-inch circle, about 3 mm thick.

Transfer the dough to the lined sheet pan. Pile the blackberry filling in the center, leaving a 5 cm / 2-inch border of dough uncovered.

Fold the dough edges up and over the filling, pleating loosely as you go, leaving the center open.

Brush the exposed crust with the beaten egg and scatter the sliced almonds over the top. Sprinkle with a little extra sugar substitute if using.

Bake for 35 to 40 minutes, until the crust is deep golden brown and the filling is bubbling thickly in the center.

Cool on the pan for at least 15 minutes before slicing. This lets the filling set so it doesn't run all over the plate.