In a large bowl, combine the whole wheat flour, coconut sugar, and salt.
2. Step
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.
3. Step
Add the ice water 1 tablespoon at a time and mix thoroughly until dough becomes crumbly and just hydrated enough to hold together.
4. Step
Dump the mixture onto a floured surface and form the dough into a disc with your hands. Cover and chill for 30 minutes.
5. Step
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.
6. Step
Preheat the oven to 375˚F (190˚C).
7. Step
On a lightly floured surface, roll out the dough into a circle about ¼- to ½-inch (6 to 13 mm) thick.
8. Step
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.
9. Step
Add the filling to the center, then fold the edges of the crust in.
10. Step
Brush the edges of the crust with egg wash and sprinkle with turbinado sugar, if desired.
11. Step
Bake for 45-50 minutes, or until golden brown.
12. Step
Serve warm or at room temperature, with ice cream, if desired.
