For the crust (makes a 9-inch single crust)
3/4 cup unbleached all-purpose flour
1/2 cup buckwheat flour
1 tsp. sugar
1/4 tsp. salt
8 tbsp. cold, unsalted butter, cut into small pieces
3 tbsp. very cold water
For the filling
1 pint strawberries, hulled, washed and thinly sliced
1 pint blueberries, washed
2 tbsp. cornstarch
2 tbsp organic cane sugar
1 vanilla bean
pinch of salt
Combine the flour, sugar and salt in the bowl of a stand mixer fitted with the paddle attachment. Mix briefly to blend. Add in the butter pieces and mix on medium-low speed to cut the butter into the flour until the mixture resembles coarse sand and the largest butter pieces are not much bigger than peas. Mix in the cold water on low speed just until the dough comes together.
Shape the dough into a ball, flatten, wrap in plastic wrap and chill in the refrigerator for at least 30 minutes, or in the freezer for 10. (The dough can be frozen for up to 2 months.)
While the dough is chilling, in a small bowl, scrape the seeds from the vanilla bean and mix with the sugar. Add a pinch of salt, cornstarch, mix, and set aside. Put the berries in a large bowl, add the sugar-cornstarch mixture and mix until combined.
Preheat the oven to 375 Fahrenheit.
Roll out the dough on a lightly floured work surface into a 12 inch circle. Place the dough on a parchment lined rimmed baking sheet (make sure you are using a rimmed sheet, because the fruit juices will seep out). Pour the berry mixture in the center of the pastry, fold back the edges to make a rim.
Bake in the preheated oven for 45-50 minutes, or until the juices start to bubble. Cool on a baking rack.