Make the galette dough. Pulse the flour, cornmeal, sugar and salt together in a food processor fitted with the blade attachment until combined. Add the butter and pulse until the mixture resembles coarse crumbs, 7 to 10 pulses. It’s okay if there are a few large pieces of butter remaining. Drizzle in two tablespoons of the ice water and pulse until dough is crumbly in texture but holds together when squeezed, about four pulses. If the mixture is dry, pulse in up to two more tablespoons of ice water, one tablespoon at a time.
Pulse the dough just to the point that it holds together when squeezed into a ball.
Place the dough into a gallon-sized zip-style bag. Flatten into a disk and seal bag. Refrigerate for one hour.
Press the dough into a thick disc and place into a zip-style bag, then refrigerate.
Toss the blackberries, sugar, flour and salt in a bowl.
Toss the blackberries with a bit of flour and sugar.
Remove the dough from the refrigerator and transfer to a floured surface. Roll the dough into a roughly 12-inch round, about an 1/8th of an inch thick.
Pile the blackberry filling into the center of the dough round, leaving at least 2 inches of dough exposed around the filling. Fold the rim of the dough up and over the edge of the blackberry filling, pleating the dough as you go and leaving the center of the blackberry filling exposed. Drizzle the exposed filling with honey and dot with small cubes of butter.
Beat egg with one tablespoon of water. Using a pasty brush, generously coat dough with egg wash. Sprinkle turbinado sugar over egg-wash-coated dough.
Brush the dough with egg wash, then sprinkle on sugar.
Bake at 375 degrees in oven or on your Traeger Grill pizza stone, lightly dusted with cornmeal, for 30 minutes or until crust is a deep, golden brown. Serve with ice cream.