How to Make It
In the bowl of a standing mixer fitted with the paddle, combine the yeast, sugar and milk. Let stand until foamy, about 5 minutes.
Add the salt, oil and water and mix at low speed for 1 minute. Gradually add about two-thirds of the flour to the bowl, beating constantly. Gradually mix in the polenta. Mix in as much of the remaining flour as needed to make the dough begin to form a ball. Continue to knead for 5 minutes at low speed. Scrape the paddle clean, cover the bowl with plastic wrap and refrigerate the dough until doubled in bulk, about 8 hours or overnight.
Remove the dough from the refrigerator, punch it down and cover with plastic wrap. Let the dough rise at room temperature until doubled in bulk, 2 to 3 hours.
Punch down the dough. Knead it on a lightly floured work surface for 30 seconds. Divide the dough in half and shape it into 2 round loaves. Cover a baking sheet or 2 round baskets of the same size with kitchen towels and sprinkle generously with flour and cornmeal. Place the 2 loaves on the towels and cover loosely with clean kitchen towels. Let the dough rise at room temperature until doubled in bulk, about 1 hour.
At least 40 minutes before baking, preheat the oven to 475°.
Lightly flour a baking sheet and turn the loaves over onto it. Slash the tops of the loaves several times with a razor blade and place in the oven. Using a plant mister filled with water, generously spray the oven roof and walls, then shut the oven door. Spray the oven three more times during the first 10 minutes of baking.
After 10 minutes, reduce the oven temperature to 400° and bake for 20 to 25 minutes more, or until the loaves are dark golden brown and sound hollow when tapped on the bottom. Rotate the loaves as necessary during baking so they brown evenly. Cool the loaves on a rack for at least 1 hour before slicing.