Step 1

Preheat oven to 425°F. Spray heavy baking sheet with nonstick spray. 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 9-inch round (the round should be about 1 inch high). Place dough on prepared baking sheet. Cut large X, 1/2 inch deep, in top of dough, almost all the way to the edges of the round.

Step 2

Bake bread in center of oven until deep brown and bottom sounds hollow when firmly tapped (a bamboo skewer inserted into the center of the bread should emerge clean without any stickiness or moistness), about 55-60 minutes. Transfer bread to rack and cool completely.