Mix sourdough starter, flour, and water until combined. Cover and ferment at room temp 8-12 hours until bubbly and doubled.

The levain is ready when it has doubled in size, has a domed top, and passes the float test.

Combine bread flour, AP flour, and water in a large bowl. Mix until no dry flour remains.

Cover and rest for 30 minutes while the levain finishes.

Add all of the levain and the salt to the autolysed dough.

Squeeze and fold the dough until the levain and salt are fully incorporated, about 3-4 minutes. The dough will become more cohesive.

After 30 minutes, perform a set of stretch and folds: grab one side of the dough, stretch it up, and fold it over to the opposite side. Rotate the bowl 90° and repeat 4 times.

Perform stretch and folds every 30 minutes for the first 2 hours (4 sets total).

After the final fold, let the dough rest undisturbed for the remaining 1-2 hours. Total bulk fermentation time is 3-4 hours. The dough should increase about 50% in volume.

Turn the dough onto a lightly floured surface. Divide into 3 equal pieces (approximately 265g each) using a bench scraper.

Preshape each piece into a loose round by folding the edges to the center and flipping seam-side down.

Cover with a clean towel and rest for 20 minutes (bench rest) to allow the gluten to relax.

Working with one piece at a time, gently flatten into a rectangle about 6 inches wide.

Fold the long edges to the center, pressing gently to seal.

Starting from one end, roll into a tight cylinder, about 14-16 inches long. Apply even pressure to create consistent thickness and good surface tension.

Place seam-side down on a well-floured couche (linen cloth), creating pleats between each baguette to support their shape during proofing.

Cover and proof for 1.5-2 hours until expanded. Use the poke test: the indent should spring back slowly but not completely.

Preheat oven to 475°F (245°C) with a baking stone on the middle rack and an empty steam pan on the bottom rack for at least 45 minutes.

Transfer baguettes to a peel or inverted baking sheet. Hold your lame at a 30-45° angle and make 3-4 swift, overlapping diagonal cuts about ¼ inch deep.

Slide baguettes onto the hot stone. Immediately pour 1 cup boiling water into the steam pan and close the door quickly.

Bake for 20-25 minutes until deep golden brown. Rotate halfway through for even browning. Internal temperature should reach 205-210°F.

Transfer to a wire rack and let cool for at least 15 minutes before slicing. The crust will crackle as it cools.

Baguettes are 14-16 inches long and cannot fit in a standard Dutch oven. Use the steam baking method instead.