Set a rack in the middle of your oven and preheat it to 450°F. Mix the flour, baking soda, and salt in a large bowl until they are fully incorporated.
Form a well in the center of the flour mixture and fill the well with the buttermilk.
Use a fork to incorporate the flour into the buttermilk little by little until a thick batter forms. Use your hands to incorporate the final bits of flour and gently shape the batter into a dough. Do not overwork the dough.
Transfer the dough to a floured work surface and gently shape it into a round 6 inches in diameter and about 1 1/2 inches thick.
Place the dough in a Dutch oven or cast iron skillet. Use a sharp knife to cut a large x into the top of the dough.
Bake for 10 minutes at 450°F. Then lower the oven temperature to 400°F and continue baking until the bread is golden brown and crusty on the outside, about 20 minutes. Transfer to a rack and cool.
*If you don't have buttermilk, simply take a cup of milk and add one tablespoon of vinegar to it to create your own buttermilk. I usually use distilled white, but any vinegar will do. You can also use citrus juice.