In a large mixing bowl combine polenta, all purpose & chickpea flours, sugar and salt. Stir until evenly mixed.
Gently stir in your egg & cold water, and using your hands, knead until a uniform dough forms.
Fold in sliced butter until evenly dispersed.
Cover, and refrigerate for at least 3 hours.
Preheat oven to 400°F. Place a pie dish or cast iron skillet into the oven to warm up while your prepare the galette.
Remove your rustic pie dough from the fridge.
On a floured or parchment paper lined surface, roll out your galette crust dough into a disc large enough to cover your pie dish/skillet with 1-2 inch overhang.
Remove your preheated dish/skillet from the oven, and gently place your dough disc into it.
Spoon your calabacitas into the center of the dough disc.
Using your fingers, gently fold the edges around the calabacitas, overlapping slightly as you go. Do not try to make this part perfect – it is a rustic pie! So long as your overlap all the way around, it will be fine.
Whisk egg and water together to form an egg wash and brush it around the crust edges. Sprinkle crust edges with coconut sugar.
Bake for 30-40 minutes, or until the crust is golden and the filling is beginning to brown.
Let cool for 10 minutes before slicing and serving.