Add the yeast to the water and let it sit for 10-15 minutes.
In a large mixing bowl, whisk together flour, sage and salt. Add mashed sweet potato and continue to mix until coated and well distributed, breaking it up with a fork if needed.
Add yeast, water and olive oil to the bowl and use a rubber spatula to mix everything together until well incorporated. The dough will appear sticky and stringy and there should still be a little flour around the edges of the bowl.
Cover and let rise somewhere warm (see note below) until it has doubled in volume, about 8-10 hours.
Using floured hands (or a dough scraper if you have one) pull the dough out onto a floured surface. Gently fold the dough into itself several times until a soft, flour-covered ball is formed. Seam side up, lift and place it into a flour dusted bowl. Dust with flour, cover and refrigerate overnight. You can even leave it for as long as 48 hours at this point if you’re not able to bake it right away.
When you're ready to bake your bread, preheat oven to 450ºF (232ºC) with your Dutch oven inside the oven.
Uncover then invert the bowl to turn the dough out onto a lightly floured surface seam side down. If needed, gently turn it with your hands and nudge it into the shape of a loaf. Place it on a sheet of parchment paper and score the top with a razor blade or the tip of a sharp knife with a design of your choosing. This will allow steam to escape while it’s baking.
Once the oven is preheated, remove the Dutch oven from the oven and transfer the dough, still on the parchment, into the Dutch oven. Cover and bake for 30 minutes, then remove the cover and bake for 15-20 minutes more. Your bread will be done once the top is browned, and it sounds hollow when you tap the top.
Using parchment edges, lift bread from Dutch oven and transfer it to a wire cooling rack to let cool for up to an hour before slicing.