Adjust oven rack to middle position and preheat to 450°F (230°C) at least 15 minutes in advance. Roughly cover bottom of deep 10-inch cast-iron or enameled Dutch oven with sheet of parchment paper; no need to trim.

Combine flour, salt, and baking soda in a large bowl and whisk for full minute to combine. Stir in buttermilk with flexible spatula until dough is fully moistened and no pockets of flour remain. For extra-fluffy results, stop folding as soon as dough comes together. For extra-chewy results, fold dough about 20 seconds more. Scrape sticky dough into prepared Dutch oven and smooth with spatula into rough boule-like shape. Score deeply into quarters with sharp knife or razor, cleaning the blade between each slice.

Cover and bake until well risen and golden, 45 minutes. Remove lid and continue baking until chestnut brown, with an internal temperature of 210°F (99°C), 12 to 15 minutes longer. Invert onto a wire rack, discard parchment, turn right side up, and cool until crumb has set, about 30 minutes. Cut thick slices to accompany hearty soups and stews, or slice thinly for sandwiches. (This will be easier if bread cools at least 2 hours.) Store up to 24 hours in an airtight container and toast to freshen bread before serving.