In a large mixing bowl, combine polenta, flour, sugar & salt. Stir until evenly mixed.
Gently stir in your egg until all is moistened, followed by your cold water, using a spatula or your hands to mix and combine all until a dough forms.
Fold in your sliced butter, knead a few times & then form your pie dough into a ball.
Cover and refrigerate overnight, or at least 4 hours.
Preheat oven to 400°F. Place a cast iron skillet or pie dish into oven to warm up while you prepare your galette.
In a large bowl, combine all filling ingredients. Toss to coat, set aside.
Remove your pie crust dough from the fridge.
On a lightly floured or parchment paper lined cutting board/countertop, flatten your dough ball into a circle using your hands.
Sprinkle dough disc with a little polenta, take a rolling pin & gently roll out into a disc/circle large enough to cover your pie dish/skillet with a 1 1/2-2 inch overhang.
Remove heated skillet/dish from preheated oven.
Gently place your rolled out pie dough into your heated dish/skillet.
Pour your berry filling evenly into center of pie dough.
Starting with the right side overhang, gently fold dough over berries followed by the bottom, overlapping each side with the next. Being a rustic pie, it is okay if it isn't perfect, just try to evenly overlap your dough over the berries all the way around.
Whisk 1 egg with 2 tbs of water to form an egg wash. Brush over edges & then sprinkle crust with 1 tbs coconut sugar.
Place galette into oven and bake for 35 minutes, or until crust is golden brown & berries are bubbling.
Let it cool & set at least 10 minutes before slicing & serving.