To make the starter, place 1/2 cup of the flour and 1/4 cup lukewarm water in a container. Stir the mixture well, cover, and let it sit at room temperature (between 70 and 75°F) overnight.
The next day, add 1/2 cup of the flour and 1/4 cup of the water to the container, mix well, cover, and let it sit at room temperature overnight. Repeat one more time.
On the fourth day, the starter is ready to use. It can be stored in the refrigerator for up to 4 days. When you are ready to make the bread, let it sit at room temperature at least 2 hours before you need it.
To make the bread, place the yeast, sugar, and 1/2 cup warm water in a small bowl. Stir until the yeast has dissolved and let it stand in a warm place for about 10 minutes, until it's frothy.
Place the yeast mixture into a large bowl, stir in the flour and the sourdough starter in small pieces. Add the 2 cups of lukewarm water and mix well.
Dust your fingers with flour. Place the dough on a cool, lightly floured surface (preferably marble), and knead it about 7 to 8 minutes. Flatten out the dough and sprinkle on the salt. Fold the dough in half and knead for 2 more minutes, until it is smooth and satiny and springs back when touched. If it is still sticky, add a little more flour and knead for one more minute.
Place the dough on a lightly floured work surface. Sprinkle a little flour over the dough, cover it with a cloth, and let it rest in a warm place (about 80 to 85°F) for 2 to 2-1/2 hours, until it doubles in size.
Put the dough back onto a lightly floured work surface and punch out the air. Divide the dough in half and knead each piece for 2 to 3 minutes. Shape each piece into a tight oval loaf.
Sprinkle flour on the loaves, cover with a cloth, and let them rest in a warm place 4 inches apart for 30 to 45 minutes, or until they double in size.
Preheat the oven to 425°F and place a pizza stone or quarry tile on the middle rack of the oven. To give the bread a nice crust, place 2 cups of boiling water in each of two deep baking pans and place them on each side of the the oven bottom about 20 minutes before baking the loaves. Or, you can mist some cold water into the oven during the first 15 minutes of baking at five-minute intervals using a spray bottle (about 4 or 5 sprays each time).
Sprinkle a baker's peel with cornmeal. Gently place the risen loaves one at a time onto the peel. Using a single-edge razor blade or the tip of a small, very sharp knife, make a few 1/2-inch-deep slashes on top of each loaf. Gently slide or place the loaves onto the pizza stone and bake for 40 minutes.
To check the loaves for doneness, tap the bottoms with your fingertip and if they don't sound hollow, bake them for 5 more minutes. When done, remove the loaves from the oven and set them on a wire rack to cool before slicing.