Set a pot with ½ inch deep water over low heat with the lid on. Clean one purple sweet potato, leaving the skin on, and chop into big 2 to 3-inch chunks. Place your chunks on a steamer basket and set the basket inside the pot, being sure that the bottom of the basket does not touch the water. Steam for about 20-25 minutes or until tender when pierced with a fork.
After you set the potatoes to steam, in a large bowl or container, combine the 275g of water and 80g of starter by whisking with a fork. Add the bread flour, all-purpose flour, and rye flour and combine thoroughly with your hand. Let sit for 30 minutes to autolyse.
When the potatoes are done, remove them from the pot and let them cool for 5 minutes. Afterwards, remove the skins, which should pretty much slide off at this point. Measure out your 150g into a small bowl and mash into a paste with a fork.
Add the 50g of water and 10g of salt to the small purple sweet potato bowl and mix with a fork. Then add the 20g of bread flour and very roughly combine, leaving the flour mostly uncombined. This mixture should be semi-dry and crumbly. If it’s noticeably wet, add another 10g of bread flour. Set aside.
When the 30 minute autolyse is over, press the dough with your fingertips to flatten it out in its vessel. Spread the potato mixture on the top of the dough. As if you were stretching and folding the dough 5-7 times, pull the dough from the bottom and fold over the potato, rotating the vessel as you work. Then, alternate between gently squeezing the dough and roughly stretching and folding the dough to loosely combine the potato mixture into the dough. It doesn’t have to be fully combined but it should be spotted and streaked with purple/small potato chunks.
Cover and set in a warm place to let bulk fermentation begin (about 4-8 hours). Perform 3 stretch and folds at the first three-hour marks. Your bulk fermentation may be longer or shorter if the dough environment runs hotter or colder/if it’s winter or summer. The optimal environment temperature for sourdough dough is around 75-82°F. You can tell when bulk fermentation is done by seeing if there are little bubbles running throughout the bottom and sides of the dough, the dough is about 30% larger and domed.
Prepare around proofing basket or line a medium-sized bowl with a rice-floured kitchen towel. Lightly flour a surface and dump the dough out onto it. If bulk fermentation went well, the dough should “burp” and deflate as you dump it out. Shape the dough into a boule and place it seam side-up in your proofing basket or bowl. Fully cover and set in the fridge overnight (12-14 hours) for cold proofing.
In the morning, remove the dough from the fridge and set it out on the counter for an hour. Preheat your oven to 450°F a half hour in, then let it the oven and your dough sit for about 15-20 minutes after preheat is over. Sprinkle the bottom of a 5 or 6-quart Dutch oven with cornmeal. Gently flip the dough into it so the floured side is face-up. Score your boule however you wish, cover with the lid, and place in the oven to bake.
After 20 minutes, remove the lid from the oven. Bake for another 40 minutes. Remove the bread from the Dutch oven and let cool for 2 hours before slicing.