2tablespoonschilled unsalted butter, cut into small pieces
Preheat the oven to 400 degrees F. Coat an 8 by 8-inch baking dish with cooking spray.
In a large bowl, toss the berries with whole wheat flour, sugar and zest. Transfer the berry mixture to the baking dish and set aside.
In a medium bowl whisk together the whole-wheat flour, all-purpose flour, 2 tablespoons of sugar, baking powder, baking soda and salt. Cut in the butter using 2 knives or a pastry cutter until many small pebble-sized pieces are formed.
In a small bowl or pitcher, whisk together the buttermilk and oil. Add the buttermilk-oil mixture to the dry ingredients and mix until just moistened. Do not over mix. Drop the batter onto the fruit forming 6 mounds. Sprinkle with the remaining teaspoon of sugar.
Bake for 30 minutes, until fruit is bubbly and top is golden. Let stand for at least 10 minutes before serving.