Heat the oven to 425°F (220°C) and line a baking sheet with parchment.
Whisk the all-purpose flour, baking soda, and fine salt together in a large bowl.
Make a well and pour in 1.5 cups of the buttermilk, mixing with a fork just until a shaggy dough forms; add the remainder only if needed to bring the dough together and leave it slightly tacky, 30–45 seconds.
Turn the dough onto the prepared sheet and gently shape it into a round about 7 inches wide and 1.5 inches thick, handling as little as possible.
With a sharp knife, cut a deep cross on top, about 0.5–0.75 inch deep, to help the center cook evenly.
Bake 15 minutes at 425°F, then reduce the oven to 400°F (200°C) and continue baking 25–30 minutes until deeply golden; the loaf should sound hollow when tapped on the bottom or read 200–205°F (93–96°C) internally.
Cool on a wire rack for 10–15 minutes before slicing; serve slightly warm for best texture.