Preheat oven to 425 degrees. Line a dutch oven with parchment paper (it can go up the sides, no need to trim!)
In a large bowl, whisk together the flours, baking soda and salt. Gradually stir in the buttermilk until the dough comes together in a ball.
Knead gently 3-4 times (you may need to add some flour to prevent sticking to your hands). Form the dough into a ball and then place in bottom of dutch oven. Flatten slightly into a larger disk, about 6" wide.
Using a small pairing knife, cut an "X" into the dough, about 1/2" deep. Place lid on top of dutch oven.
Bake for 20-25 minutes, covered, then remove the top pan and bake uncovered for about 20 minutes more or until the crust is golden brown and crispy.