Preheat oven to 400-degrees, and line a large sheet pan with parchment paper. In a large bowl, combine flours, baking soda and salt; whisk to lighten and remove any lumps. Make a well in the center of the dry ingredients and pour in the buttermilk.
Gently mix with a rubber spatula until combined; do not knead or overwork the dough. Turn dough out onto prepared pan. Gently form into an even and round loaf and cut a shallow “x” pattern across the center using a serrated knife. Bake for 25 to 30 min or until firm and lightly golden. Allow to cool for 10 to 15 minutes before slicing. Makes one loaf, can be sliced into 12 large or 24 small slices.