Preparation

Make the filling

• In a large bowl, toss together the blueberries, blackberries, raspberries, granulated sugar, salt, lemon juice, and cornstarch.  Transfer to a 10 by 14-inch rectangular, ceramic baking dish.

Make the dough and bake the cobbler

• Set the oven rack in the lower third of the oven. Preheat the oven to 375°F. Line a baking sheet with parchment paper or a nonstick silicone baking mat.
• In a large bowl, whisk together the flour, granulated sugar, baking powder, baking soda, salt, and crystallized ginger.
• Working quickly so as not to warm the butter, work the butter into the flour mixture with your fingers until it resembles coarse crumbs. Add 6 Tbs. of the buttermilk and fold with a rubber scraper or your hands until the buttermilk has been absorbed and there are no dry patches. Add the remaining 2 Tbs. of buttermilk as necessary (you won’t use all of it; reserve what’s left for brushing the dough). The dough should be dry enough to hold a shape when you cut it out.
• Turn the dough out onto a lightly floured work surface. With lightly floured hands, pat the dough to about 3/4 inch thick. Cut out twelve stars with a 3-3/8-inch (measuring point to point) star cookie cutter and place them in a single layer on top of the fruit. Brush the stars with the reserved buttermilk, and sprinkle each with 1/4 tsp. of sanding sugar. Bake, rotating the baking sheet two-thirds of the way through the baking time, until the biscuit topping is lightly browned and cooked through, 60 to 65 minutes. Serve warm or at room temperature.