To prepare berry mixture: Combine berries, lemon juice and zest, sugar and corn starch in a bowl and toss to coat evenly. Transfer berry mixture to a 10-inch cake pan or cast iron skillet. Set aside. To prepare the biscuits: Stir together flours, baking powder and soda, salt, black pepper. Cut in butter and shortening until mixture resembles coarse crumbs. Add buttermilk and mix dough until it comes together. Drop biscuit dough in 1/3 cup portions and arrange in cake pan on top of berry filling. Brush the tops of the biscuits with melted butter and sprinkle with sea salt and freshly cracked black pepper. Bake at 400° F for 30-35 minutes or until golden brown. Let cool at least 20-30 minutes before serving.