In a bowl of stand mixer, combine the flour, water, milk, and biga. Using the dough hook,mix on low speed until everything blended.
Add the salt and yeast and mix on low speed for 5 minutes. Increase the speed to medium and mix for about 7 more minutes or until the dough is smooth. When the gluten is fully developed, mix in the olive oil and rosemary on low speed.
Lightly oil a large bowl. Scrape the dough into the bowl and cover with plastic wrap.
Let the dough ferment for 7 hours.
Remove the dough to a lightly-floured work surface and divide it into four (or two if you halved the recipe) 450-gram /16-ounce pieces. Shape the dough pieces into rounds. Cover with plastic wrap and let them bench rest for 15 minutes.Uncover the dough and, if necessary, lightly flour the work surface. Gently press on the dough to degas and carefully shape each piece into tight and neat rounds.
Place one loaf on one side of the baking sheet lined with parchment paper a double layer of cloth to serve as a divider between the loaves, and place a second loaf next to the fold. Repeat the process with the remaining two loaves. Cover with plastic wrap and proof for 1 hour.
About an hour before you plan to bake the loaves, place a baking sheet (stones) into the oven along with a steam pan (underneath) or iron skillet (on the top rack) and preheat the oven to 450 degrees F.
Uncover the dough and score the top of each loaf in a star pattern using a lame or sharp knife.
Optional: sprinkle sea salt into the crevices as the original baker did to make it "sparkle with diamonds."
To make the steam, add 1 cup of ice to the iron skillet or steam pan kept at bottom of oven
Bake for 30 minutes, or until the crust is light brown and crisp and the loaves make a hollow sound when tapped on the bottom. Loaves should register an internal temperature of 200F.
Remove the loaves from the oven and transfer to a wire rack to cool.