Step 1

Activate your starter: Mix the unfed starter, water and all-purpose flour in a small container. Let your starter ferment until doubled or tripled in volume (about 4 to 6 hours).

Step 2

Mix your dough: In a large bowl, combine water and active sourdough starter. Stir until the starter is almost completely dissolved. Fold the whole wheat flour into the starter mixture. Add all of the bread flour and salt. Stir using a wooden spoon or your hands until no dry traces of flour remain.

Step 3

Fold the dough: Complete a set of stretch and folds by picking up one side of the dough and folding it over itself. Repeat until all sides are folded. Transfer the dough to a lightly-oiled bowl, cover with plastic wrap and let the dough rest for 1 hour.

Step 4

Strengthen the dough and bulk ferment: As your dough ferments, perform 3 to 4 sets of coil folds inside the bowl at 30 minute intervals, cover your bowl with plastic wrap after you complete each fold. Your dough should appear smooth and pass the windowpane test after the last fold.

Step 5

Bench rest: After your last fold, let the dough rest for 1 hour untouched in a warm place. Your dough should look puffy and should increase in volume during this period.

Step 6

Shape the dough: Prepare your proofing basket (you can use a small mixing bowl lined with a tea towel dusted with rice flour).

Step 7

Turn your dough out on a lightly-floured work surface. Lightly flatten the dough out into a rectangle (be gentle so you do not pop the air bubbles build up during fermentation).

Step 8

Fold the top of the dough towards the center and then fold the bottom of the dough over that, like you are folding a letter. Pick up the sides of the dough, fold it in half and place it in your proofing basket, seam-side up. Pinch the seam shut. Fold the tea towel over your dough.

Step 9

Cold proof: Place your dough inside the refrigerator and proof overnight.

Step 10

Prepare for baking: Preheat your oven to 500 F with a Dutch oven inside for at least an hour.

Step 11

Score: Turn your dough out on a piece of parchment paper. Cut a long slash with a sharp knife or a lame on the dough to let the steam escape.

Step 12

Bake: Place the dough with the parchment paper into the Dutch oven, turn. Bake for 30 minutes with the cover on and an additional 10 to 15 minutes with the cover off, depending on how dark you would like your crust.

Step 13

Serve: Transfer your baked loaf to a cooling rack. Let the dough cool completely, at least 2 hours, before slicing.

Step 14

Store: Store any leftover slices in a ziplock bag for up to 5 days at room temperature.