Mix 1 cup bread flour, 1/4 teaspoon yeast and 2/3 cup warm water in a large bowl. Cover with plastic wrap and set aside until it rises and looks very bubbly, 2 hours.
Meanwhile, prick the sweet potato in a few places with a knife and microwave until tender, 8 to 10 minutes. Let cool slightly. Peel the potato and transfer to a food processor along with the olive oil and pinch of salt; puree until very smooth. (You should have about 1 cup puree; if you have more than 1 cup, reserve the extra for another use.)
Stir the yeast mixture with a spoon to deflate it, then add the remaining 2/3 cup warm water and 1 1/2 teaspoons yeast and stir. Add 2 1/2 more cups bread flour, the sage, sugar and remaining 2 1/2 teaspoons salt; mix until the dough is shaggy and there is no dry flour in the bowl. Let stand until the flour absorbs some of the liquid, 5 minutes.
Put the remaining 3/4 cup flour in a small dish for kneading. Lightly dust a work surface with some of the flour and turn out the dough onto the surface. Lightly flour the top of the dough. Knead the dough, adding a little bit of the flour as needed, until very smooth, elastic and a little tacky, about 5 minutes. (Alternatively, you can knead the dough in a stand mixer with the dough hook on medium speed for about 5 minutes.)
Generously flour the surface under the dough, then pat the dough into a rough square. Top with about half of the sweet potato puree. Pull and fold the edges of the dough over the puree and then begin to knead together (it will get messy at first but the dough will come together). Add more flour as needed to knead and swirl the puree into the dough, about 1 minute (use a dough scraper or butter knife to scrape up any puree that comes out and add it back to the dough). Pat the dough flat and add the remaining puree. Keep kneading the dough, using the remaining flour, until the puree is mixed in but not fully blended and the dough comes back into a ball, 2 to 5 minutes (it’s OK if the surface is a little rough). Put smooth-side up in a lightly oiled large bowl, turning to coat. Cover the bowl with plastic wrap and let rise until doubled in size, 1 to 2 hours.
Gently turn out the dough onto a lightly floured surface. Gently press the dough to deflate it. Stretch and pull the edges into the middle of the dough, starting with two sides, then two other sides; repeat until the dough looks like a round. Using a bench scraper, turn over the dough onto a spot with little to no flour. Cup your hands around the dough and gently pull the dough toward you about 1 inch. Rotate the dough a bit and gently pull toward you again. Continue with five to seven pulls. The dough should sit a bit taller than before and the top should feel taut.
Line a large bowl with a clean towel and dust with flour. Add the dough seam-side up, dust with flour, then cover with plastic wrap. Set aside until it’s puffy and rises by about 50 percent, about 1 hour. (If the temperature is higher than 75˚ F, let rise in the refrigerator.)
Place a large Dutch oven on the middle oven rack; preheat to 450˚ F. Uncover the dough and brush off any excess flour from the bottom. Dust the top with cornmeal. Put a piece of parchment against the dough and then carefully flip it out onto the parchment. Trim the parchment so it fits inside the Dutch oven, leaving enough parchment around the dough for you to lift it. Carefully remove the Dutch oven from the oven and transfer the dough to the hot pot using the excess parchment. Cut a long slash in the top of the dough with a sharp knife, about 1/4 inch deep.
Cover the pot and bake the bread 15 minutes. Uncover and bake until the bread is golden brown and feels hard and hollow when tapped, 15 to 20 more minutes. Let the bread cool in the pot, 5 to 10 minutes. Using a large metal spatula, transfer the bread to a rack to cool, at least 1 hour; the crust will soften slightly.