Preheat your oven to 425°F (220°C). Line a baking sheet with parchment paper, or lightly grease a cast iron skillet or Dutch oven.
In a large bowl, whisk together the flour, baking soda, and salt. If using additional ingredients, stir them into the flour mixture.
Make a well in the center of the dry ingredients and pour in the sour milk (or buttermilk). Using a wooden spoon or your hands, mix until a sticky dough forms. Turn the dough out onto a lightly floured surface and knead it briefly until it comes together into a round loaf.
Place the dough onto the prepared baking sheet, cast iron skillet, or Dutch oven. Use a sharp knife to score a deep cross on the top of the bread.
Bake in the preheated oven for 35-40 minutes or until the bread is golden brown and sounds hollow when tapped on the bottom.
Transfer the bread to a wire rack to cool completely before slicing.