Start this at night to serve the next day, and expect up to 2 hours of rising time for the dough.
In a large bowl, mix all the ingredients for the overnight sponge. Knead it a little to incorporate everything if it’s too stiff for the spoon, but the result should be a soft dough, not a firm ball. Dribble a little oil over it to keep it from forming a skin overnight – smooth it with your hand to make a thin film over the dough. Cover the bowl with plastic wrap and put it away in a cool place overnight.
By the next morning, the sponge should be light, pocked with bubbles, and have a pleasantly sour smell. Now get a small bowl ready, and measure the remaining 1/4 cup water into it. Add the baking soda and the salt. Mix. Add the oil to the mixture.
Pour the water mixture into the sponge and beat it in.
Add about 3 cups more of flour, kneading it in. If the dough is too soft to handle, add flour a little more at a time. Stretch it out between your hands and fold it as you would make a paper envelope – then stretch it out and fold again, three or four times more. It may get sticky – just dust a little flour over your hands to keep going. You don’t want a stiff dough, but one just cohesive enough to handle.
Shape your loaves, cover them, and let them rise in a warm place 1 1/2-2 hours or until they’ve grown light, with noticeable bubbles under the surface skin.
Preheat the oven about 20 minutes before you anticipate baking.
It’s best to slash the tops before baking, to prevent a cracked crust. So cut some slashes into them and then allow them to recover for 5 minutes.
Bake at 350° F (180° C) for 30 minutes.
Inspect the loaves for done-ness. If they seem a bit under-baked, turn them upside down and continue baking for another 5-10 minutes.
This bread tastes best when totally cooled down. That gives the sweet flavors of wheat and corn a chance to come through.
Sourdough cornmeal bread from Unpacked – https://jewishunpacked.com/sourdough-cornmeal-bread/