Mrs. O'Callaghan's Soda Bread

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 7-inch round. Place dough on prepared baking sheet. Cut large X, 1/2 inch deep, in top of dough. Bake bread until deep brown and bottom sounds hollow when firmly tapped, about 40 minutes. Transfer bread to rack and cool completely.