Preheat oven to 350 degrees. Butter an 8 1/2 x 4 1/2 x 2 1/2-inch bread pan. In a large bowl, mix together flours, steel-cut oats, rolled oats, brown sugar, salt and sifted baking soda. Mix well with your hands. Make a well in the center of flour mixture. Pour in buttermilk. Working from the center of the bowl in concentric clockwise circles, with fingers outstretched, stir buttermilk into flour mixture. (You can use a rubber spatula instead if you don’t like getting dough on your hands.) This should take about a half a minute at most. Dough will be quite moist. Use a rubber spatula to scrape into bread pan and smooth out the dough to fill pan evenly ( the pan will be filled only about halfway.) Place in the oven and bake 40 minutes, until dark brown and a tester inserted comes out clean. Remove from pan and cool on a rack.