Preheat the oven to 450F. Line a cast iron skillet (or use another oven-proof skillet or a baking sheet) with parchment paper.
In a large mixing bowl, whisk together the yogurt and milk. Stir in the oats and flax and let it soak for at least 10 minutes.
In a medium bowl, mix together the flours, salt, and baking soda.
Add the dry ingredients to the wet ingredients and use a rubber spatula to mix until it just comes together. Then use your hands to gently form a ball. Place in the center of the skillet and flatten it slightly to about 7 or 8-inches diameter. Use a sharp knife or bread lame to score (make a shallow cut) the dough in an “X” about ½-inch deep.
Bake for 15 minutes, then reduce the oven to 400F and bake for 30-35 more minutes or until a knife inserted into the center comes out clean.
To be sure that the bread is cooked, insert an instant read thermometer into the thickest part of the loaf (the center). The internal temperature should be about 190F. If it’s not, then bake it for 5-10 more minutes.
Let it cool for at least 30 minutes before slicing, but up to 4 hours for best results (it slices better once cooled).