For the dough
3/4 cup all purpose flour
1/2 cup whole wheat flour
pinch of salt
1 1/4 stick unsalted butter, cut into cubes
3 tbl. very cold water
For the egg wash
1 egg, slightly beaten
1 tbl. water
For the filling
1 cup blackberries
1 cup rhubarb, diced into 1-inch segments
1/4 cup sugar
2 tbsp. flour
1/2 tsp. ground cinnamon
1/2 tsp. ground ginger
In a large bowl, whisk together the flours and salt. Add in the cold butter and toss until butter is covered in flour. Using a pastry cutter (or your fingers), mash the butter into the flour mixture until the butter is the size of peas.
Add in the water, a little bit at a time, until the dough mixture is just combined. Be careful not to add too much water or overmix. Shape the dough into a ball and chill in the fridge for 30 minutes - 1 hour.
Preheat the oven to 425F. In a medium-sized bowl, combine the blackberries, rhubarb, sugar, flour, and spices. Toss until thoroughly coated.
Remove dough from fridge, and roll it out. Pour the filling mixture in the center of the galette, leaving a 2 inch edge around the filling mixture.
Fold the dough over the edge of the fruit and crimp. Brush the egg wash on top and sprinkle with turbinado sugar (optional).
Bake in preheated oven for 20 - 25 minutes, or until crust is golden brown. Slice and enjoy!