For the Sourdough Levain

• 2 tablespoons active sourdough starter
• 2 tablespoons bread flour
• 2 scant tablespoons water

For the Bread Dough

• 1 3/ 4 cups bread flour
• 1/ 3 cup whole wheat flour
• 1 cup plus 2 tablespoons water
• 1 1/ 4 teaspoons sea salt
• 1 purple sweet potato, cubed

Method

• Step 1

  To make the levain: Mix together starter, flour and water in a bowl. Set aside for 4-6 hours.

• Step 2

  When there's 2 hours left for the levain, mix the flours and water for the dough in a large bowl until the flour is just hydrated. Let rest, covered, at room temperature for 2 hours. This is the autolyse period.

• Step 3

  Halfway through the autolyse period, bring a pot of water to a boil. Steam the sweet potato for 15-20 minutes, until soft enough to insert a knife with no resistance. Transfer potato to a bowl, mash, and let cool, about 30-45 minutes.

• Step 4

  Mix the levain and autolysed flour together for 2-3 minutes, until fully incorporated. Cover and let rest for 30 minutes.

• Step 5

  Add salt to dough mixture and mix for 3-4 minutes by hand (less time if using a mixer). Divide the dough into 2 equal portions.

• Step 6

  Lightly mist a counter top with water. Spread out half the dough until it creates a 10 x 10-inch square. Spread the mashed potatoes in a thin layer on top. Fold the dough over on itself, give it a small knead, then repeat, incorporating the potato into the dough. Shape into a ball. Cover each ball of dough separately and set aside to rest for 45 minutes.

• Step 7

  On a lightly water-misted counter, put the sweet potato dough on top of the plain dough. Gently stretch both doughs out as one, until forms a 20 x 20-inch square. Create a letter-fold by folding the dough into thirds, starting from the left or right edge. Repeat the letter-fold with the top and bottom edges of the dough. Transfer to an 8 x 8-inch glass baking dish, cover and let rest for 1 hour.

• Step 8

  Pull the dough from the side closest to you, moving up toward your body, so that the furthest edge pulls away from the bowl and folds back in on itself underneath. Put the dough back into the bowl, turn 180º and repeat. Then turn 90º, repeat, and turn 180º to repeat the fold on the last side of the dough. If your dough feels very wet, you may need to repeat this process more times to create enough strength in the dough.

• Step 9

  Tip the dough onto a well-floured surface. Carefully stretch out the sides of the dough to do a letter-fold. Starting from the top of the dough, gently roll the dough into a log, sealing at the bottom edge. Sprinkle with brown rice flour and carefully flip upside down into a 10 x 6 x 4-inch rattan basket. Cover and refrigerate overnight, for about 12-16 hours.

• Step 10

  Place a Dutch oven into the oven to preheat to 485ºF for 45-60 minutes.

• Step 11

  When the oven is ready, remove dough from fridge and sprinkle with more brown rice flour. Transfer to a piece of parchment paper. Slash the top of the dough with a razor blade or serrated knife. Transfer the dough, on the parchment paper, to the preheated dutch oven.

• Step 12

  Bake covered for 20 minutes, then remove lid and reduce oven temperature to 450ºF. Continue baking for 20-25 minutes, until well-browned. Cool at least 2 hours before slicing.