When added to the dough, the poolish's complex fermented flavors enrich the french bread's flavor.
In the bowl of an electric mixer fitted with a paddles attachment, add the yeast, water, sourdough starter; blend together.
In a bowl, whisk together 4 cups flour, sugar, and salt. Add to the sourdough starter mixture; blend for about 3 minutes.
Add a couple of tablespoons of oil to a large bowl, turn dough into the bowl and coat all sides with the oil; cover with a clean dish towel; set aside in a warm, draft-free place for
1 1/2 - 2 hours or until doubled in volume.
In a bowl, whisk together 2 cups flour with the baking soda.
Return the dough to the bowl of an electric mixer fitted with a dough hook; gradually add the flour/baking soda mixture until the dough cannot absorb any more flour; knead the dough until smooth and elastic, about 5 - 7 minutes.
Separate the dough into two pieces. On a lightly floured silicone baking mat, roll out one piece of dough into a rectangle; starting at one long end, roll up into a cylinder, place on a silicone baking mat sprinkled lightly with cornmeal. Repeat with the other piece of dough. Cover and place in a warm, draft-free place for 1 1/2 - 2 hours, until nearly doubled in volume.
Preheat the oven to 400ºF. when the loaves have about 30 minutes left to rise. Just before baking, brush the loaves with water. Make diagonal slits in the loaves.
Place a shallow pan of hot water in the bottom of the oven. Bake for 45 or 50 minutes, until the crust is golden brown.