1. Make the biga: Stir together the flour and yeast in a 4-quart bowl (or in the bowl of an electric mixer). Add ¾ cup plus 2 Tablespoons of water, stirring until everything comes together and makes a coarse ball (or mix on low speed for 1 minute with the paddle attachment). Adjust the flour or water, according to need, so that the dough is neither too sticky nor too stiff. (It is better to err on the sticky side, as you can adjust easier during kneading. It is harder to add water once the dough firms up.)
Sprinkle some flour on the counter and transfer the dough to the counter. Knead for 4 to 6 minutes (or mix on medium speed with the dough hook for 4 minutes), or until the dough is soft and pliable, tacky but not sticky. The internal temperature should be 77° to 81°F.
Lightly oil a bowl and transfer the dough to the bowl, rolling it around to coat it with oil. Cover the bowl with plastic wrap and ferment at room temperature for 2 to 4 hours, or until it nearly doubles in size.
Remove the dough from the bowl, knead it lightly to degas, and return it to the bowl, covering the bowl with plastic wrap. Place the bowl in the refrigerator overnight. You can keep this in the refrigerator for up to 3 days, or freeze it in an airtight plastic bag for up to 3 months.
2. Remove the biga from the refrigerator 1 hour before you plan to make the bread. Cut it into about 10 small pieces with a pastry scraper or serrated knife. Cover with a towel or plastic wrap and let sit for 1 hour to take off the chill.
3. Stir together the flour, salt, black pepper, and yeast into a 4-quart mixing bowl (or in the bowl of an electric mixer). Add the biga pieces, mashed potatoes, oil, rosemary, and ¾ cup plus 2 Tablespoons water. Stir with a large spoon (or mix on low speed with the paddle attachment) for 1 minute, or until the ingredients form a ball. Add more water, if necessary, or more flour, if the dough is too sticky.
4. Sprinkle flour on the counter, transfer the dough to the counter, and begin to knead (or mix on medium speed with the dough hook). Knead for approximately 10 minutes (or 6 minutes by machine), adding more flour if needed, until the dough is soft and supple, tacky but not sticky. It should pass the windowpane test and register 77° to 81°F. Flatten the dough and spread the roasted garlic over the top. Gather the dough into a ball and knead it by hand for 1 minute (you will probably have to dust it with flour first to absorb the moisture from the garlic). Lightly oil a large bowl and transfer the dough to the bowl, rolling it around to coat it with oil. Cover the bowl with plastic wrap.
5. Ferment at room temperature for approximately 2 hours, or until the dough doubles in size.
6. Remove the dough from the bowl and divide it into 2 equal pieces for loaves, or 18 equal pieces (about 2 ounces each) for dinner rolls. Shape each of the larger pieces into a boule, or shape the smaller pieces into rolls. Line a sheet pan with baking parchment (use 2 pans for rolls) and dust lightly with semolina flour or cornmeal. Place the dough on the parchment, separating the pieces so that they will not touch, even after they rise. Mist the dough with spray oil and cover loosely with plastic wrap.
7. Proof at room temperature for 1 to 2 hours (depending on the size of the pieces), or until the dough doubles in size.
8. Preheat the oven to 400°F with the oven rack on the middle shelf. Remove the plastic from the dough and lightly brush the breads or rolls with olive oil. You do not need to score these breads, but you can if you prefer.
9. Place the pan(s) in the oven. Bake the loaves for 20 minutes, then rotate the pan 180 degrees for even baking. The loaves will take 35 to 45 minutes total to bake. Bake the rolls for 10 minutes, rotate the pans, and then bake for 10 minutes longer. The loaves and rolls will be a rich golden brown all around, and the internal temperature should register at least 195°F. The loaves should make a hollow sound when thumped on the bottom. If the loaves or rolls are fully colored but seem too soft, turn off the oven and let them bake for an additional 5 to 10 minutes to firm up.
10. Remove the finished loaves or rolls from the oven and cool on a rack for at least 1 hour for loaves and 20 minutes for rolls before serving.