Nonstick vegetable oil spray
3 cups all purpose flour
3 cups whole wheat flour
1/2 cup (packed) brown sugar
1 teaspoon baking soda
1/4 cup (1/2 stick) chilled margarine or butter, cut into 1/2-inch cubes
2 cups buttermilk (add additional, about 1/2 cup, to get the dough to shaggy)
Preheat oven to 425°F.
Spray heavy baking sheet with nonstick spray. (I used a pizza stone.)
Whisk both flours, sugar, and baking soda in medium bowl to blend. Add margarine and cut in until margarine is reduced to pea-size pieces. Add buttermilk; stir until shaggy dough forms.
Turn dough out onto lightly floured work surface. Knead until dough comes together, about 10 turns. Shape dough into 7-inch round. Place dough on prepared baking sheet. Cut large X, 1/2 inch deep, in top of dough. (I made two six inch loaves instead of one.)
Bake bread until deep brown and bottom sounds hollow when firmly tapped, about 40 minutes. Transfer bread to rack and cool completely.