Preheat oven to 400 degrees F (200 degrees C). Prepare a shallow baking pan with cooking spray.
Sift together the bread flour, whole wheat flour, salt, baking soda, and 2 teaspoons sugar in a bowl. Cut the margarine into the flour mixture until pieces are nearly indistinguishable. Make a well in the center of the mixture and pour in the oil and buttermilk. Stir with a spatula until dry mixture is completely moistened. Move the dough to a lightly-floured surface. Lightly knead the dough for no more than 1 minute. Place the dough into the prepared pan; pat down and around to form a round loaf. Cut a cross into the top of the loaf with your finger. Brush the top with 1 tablespoon buttermilk; sprinkle 1 teaspoon sugar over the top of the loaf.
Bake in the preheated oven for 30 minutes. Reduce heat to 375 degrees F (190 degrees C); rotate pan and bake another 30 minutes.