Prepare the Biga

1. In a mixing bowl, combine the bread flour, water, and a pinch of instant yeast. Stir with a wooden spoon or Danish dough whisk until well blended.
2. Scrape down the sides of the bowl, cover with plastic wrap, and let it rest at room temperature (about 75°F/24°C) for 14 to 16 hours.

Make the Final Dough

1. In the bowl of a stand mixer, combine the bread flour, whole wheat flour, and a pinch of yeast.
2. Add water, milk, and all of the biga. Using the dough hook, mix on low speed until everything is blended.
3. Add salt and mix on low speed for 5 minutes. Increase the speed to medium and mix for an additional 7 minutes, or until the dough is smooth and elastic.
4. On low speed, gradually mix in the olive oil and chopped rosemary until evenly distributed.
5. Lightly oil a large bowl. Transfer the dough to the bowl, cover with plastic wrap, and let it ferment at room temperature for about 7 hours.
6. Turn the dough out onto a lightly floured surface. Divide into four equal pieces (450 grams each). Shape each piece into a tight round.
7. Cover the dough rounds with plastic wrap and let them rest for 15 minutes.
8. Degas the dough gently and shape into final rounds.
9. Arrange the loaves on a parchment-lined baking sheet. Cover and let proof for 1 hour.
10. About an hour before baking, place a baking stone and a steam pan in the oven. Preheat to 450°F (232°C).
11. Uncover the dough. Score the top of each loaf in a star pattern. Add ice to the steam pan. Bake for 30 minutes, or until golden brown and hollow sounding when tapped.
12. Transfer loaves to a wire rack to cool completely.