Stir the flour and yeast together in a bowl (or in the bowl of a stand mixer). Add 6 tablespoons of the water and stir until everything comes together and forms a coarse ball. Ad more flour or water if needed to form a ball that is not too sticky, but also not too stiff.

Sprinkle flour on a work surface and transfer the dough to the counter. Knead for 4 to 6 minutes, until the dough is soft and pliable. (Alternately, you can use the dough hook for 4 minutes.)

Lightly oil a large bowl, then transfer the dough to the bowl and roll around to coat with the oil. Cover the bowl with plastic wrap and allow the dough to sit at room temperature until it doubles in size.

Remove the dough from the bowl and knead lightly to degas the dough. Return it to the bowl, cover with plastic wrap, and place the bowl in the refrigerator. Refrigerate overnight. (Dough can be kept in the refrigerator for up to 3 days.)

Remove the biga from the refrigerator 1 hour before starting the bread. Cut it into about 10 smaller pieces, cover with a towel, and let it sit for 1 hour.

In a large bowl (or in the bowl of your stand mixer) stir together the flour, salt, black pepper and yeast. Add the biga pieces, mashed potatoes, oil, rosemary and 3/4 cup plus 2 tablespoons of the water. Stir (or mix on low speed) for 1 minute, until the ingredients form a ball. Add more water or flour, if necessary, to form a dough that is not too sticky or stiff.

Transfer the dough to a lightly floured counter. Knead for 10 minutes, adding flour as needed, until the dough is soft and supple. (Alternately, mix in medium in a mixer with the dough hook.) Flatten the dough and spread the roasted garlic over the top. Take the ends up into a ball, enclosing the side of the bread with the garlic, then knead the bread for 1 minute, working the garlic into the bread. (You may need to add a little bit more flour to absorb the moisture from the garlic.)

Lightly oil a large bowl and add the dough, turning to coat. Cover with plastic wrap and let the dough rise until doubled, about 2 hours.

Remove the dough from the bowl and divide it into 2 equal sized balls. Shape each of the balls into a boule. Line a sheet pan with parchment paper or a silpat liner and sprinkle lightly with cornmeal. Place each of the boules on the sheet pan, separating so that they won’t touch, even after rising. Mist the dough with nonstick cooking spray and cover lightly with plastic wrap. Allow to proof at room temperature for 1 to 2 hours, until the dough doubles in size.

Preheat the oven to 400ºF with the oven rack in the middle of the oven. Remove the plastic wrap from the dough and lightly brush the bread with olive oil. If desired, you can score the dough.

Bake the loaves for 20 minutes, then turn the sheet pan 180º. Bake an additional 15-25 minutes. The loaves should be golden brown and should make a hollow sound when thumped on the bottom.

Remove the loaves from the oven and cool for at least 1 hour before serving.