Boil Potatoes: Start by boiling your potatoes until soft. Mash them into a puree and let cool.
⅓ cup potato puree
Begin Autolysis: Mix the flour with ⅔ of the water to start the autolysis process. Cover the mixture with plastic wrap and let it rest for about 3 hours.
2 ⅓ cups semolina flour, ¾ cup water
Combine Ingredients: In your mixer's bowl, combine the autolyzed flour, potato puree, sourdough starter, and the remaining water. Use the dough hook and mix for about 15 minutes until you get a smooth dough. Add salt, mix until well incorporated, then transfer the dough to a work surface.
⅓ cup sourdough starter, 2 teaspoons salt
Knead Dough: Knead the dough with your fists for a couple of minutes until compact. Cover with an inverted bowl and let it rest for 30 minutes.
Fold Dough: After resting, flatten the dough into a square, fold it over itself in thirds, cover, and let rest again for 30 minutes. Repeat the folding process, cover, and let it rest for another hour and a half, with a final 15-minute rest after the last set of folds.
Shape and Proof: Shape the dough into a tight ball. Place it in a basket or bowl lined with a clean, semolina-dusted cloth. Cover with the cloth and then plastic wrap. Let it sit at room temperature for 1 hour, then refrigerate for 18 hours.
Bake: Bring the dough to room temperature and let it rise for about 2 hours. Once doubled, score the top, then bake at 425°F with a small pan of water in the oven for the first 20 minutes. Remove the pan and bake for another 20 minutes or until the bottom sounds hollow when tapped. Cool before slicing.