In a large bowl, combine the whole wheat flour, coconut sugar, and salt.
Add the butter and use a pastry cutter to work it into the flour mixture, until the butter becomes about the size of peas. Work quickly to keep the dough cold.
Add the ice water 1 tablespoon at a time and mix thoroughly until dough becomes crumbly and just hydrated enough to hold together.
Dump the mixture onto a floured surface and form the dough into a disc with your hands. Cover and chill for 30 minutes.
Meanwhile, make the filling: combine the peaches, blueberries, lemon zest, coconut sugar, cornstarch, cinnamon, cardamom, and vanilla in a large bowl and mix well. Cover and chill while the dough chills.
Preheat the oven to 375˚F (190˚C).
On a lightly floured surface, roll out the dough into a circle about ¼- to ½-inch (6 to 13 mm) thick.
Carefully use the rolling pin to transfer the dough to a 10- or 12-inch (25 cm or 30 cm) cast-iron skillet. Gently press the dough against the bottom and sides of the pan.
Add the filling to the center, then fold the edges of the crust in.
Brush the edges of the crust with egg wash and sprinkle with turbinado sugar, if desired.
Bake for 45-50 minutes, or until golden brown.
Serve warm or at room temperature, with ice cream, if desired.