Prepare overnight levain

Combine all of the ingredients in a small mixing bowl. Beat with a spoon or whisk for about a minute, until the batter is mixed thoroughly and is fairly smooth. Cover the bowl with plastic wrap. Let it rest on the counter for about 12-14 hours, overnight. During this time, it should expand and become bubbly.

Mix the final dough

In a separate mixing bowl, add the all-purpose flour, lemon zest and chopped thyme. Pour in 325 grams of water. Mix in additional water, 10 grams at a time, if needed until there are no dry bits of flour. Cover, and let the flour/water mixture rest (autolyze) for 30 minutes.

Add the levain and dried yeast, if using, and mix thoroughly using your hands or a bowl scraper. Cover and let the dough rest for 20 minutes.

Sprinkle the salt over the dough and use wet fingers to incorporate it throughout the dough. Dip your fingers in water, while mixing, if necessary. The dough should be soft and somewhat smooth, although the surface may still look a bit rough.

Cover, and let the dough rise for 45 minutes. Deflate the dough and gently stretch and fold the edges into the center. Place it seam-side down in the bowl, cover and let it rise for an additional 45 minutes. The dough should be noticeably puffy.

Shape the baguettes

Divide the dough into two equal portions, and shape them into rough rounds. Place each piece seam side down on a lightly floured work surface. Cover with a kitchen towel and let them rest for 20 minutes.

Working with one piece at a time, place the dough seam side up and gently press to degas it. Form into a rough rectangle. Fold the top edge of the rectangle down toward the center. Press firmly along the fold with your fingertips to seal and degas. Rotate the dough 180° so the bottom edge becomes the top. Fold the new top edge down toward the center, pressing firmly to seal.

Fold the top of the dough down a couple of more times and seal it with your fingers or the heel of your hand. This will help to degas and get a tight seal. You should end up with a rough batard-shaped loaf.

Place the dough seam-side down. Using both hands, gently roll the dough into a 15-inch long, even cylinder, starting at the center and working outward. Lightly taper the ends by applying gentle pressure as you roll. Ensure the seam is sealed tightly, and place the shaped baguettes seam-side up in lined baguette proofing baskets.

Cover, and let the loaves rise in the banneton baskets for 45 minutes to 1 hour.

Preheat the oven

30-45 minutes before you plan to bake, prepare your oven for baking by placing one baking stone on the lower rack and another baking stone on the upper middle rack. If you’re only using one baking stone, place it on the middle rack.

Place a cast iron skillet, or other steam pan, on the bottom rack or floor of your oven. Preheat the oven to 450 degrees F.

Score & bake the baguettes

Gently flip the proofed baguettes from the proofing baskets seam-side down onto a parchment-lined baking sheet.

Score the tops of the baguettes with a bread lame or sharp knife. Holding the lame at a 45-degree angle, make one long slash down the center of each baguette.

Carefully slide the loaves off of the baking sheet (parchment sheet and all) and onto the baking stone on the bottom baking stone. Immediately pour 1/4 cup boiling water into the steam pan, and close the door.

Bake the loaves for 12 minutes. Rotate and move the loaves up to the top baking stone. Pour another 1/4 cup boiling water into the steam pan, and close the door. Bake for 8-10 minutes longer, or until they are golden brown. Total bake time 20-25 minutes.

If you are only using one baking stone, bake the loaves on the middle shelf for about 24-28 minutes, or until they are golden brown, and rotate the loaves halfway through the baking cycle for even baking.

Transfer the loaves to wire racks and allow them to cool for at least 30 minutes before slicing and eating.

Store the baguettes

Store any leftover baguette in a paper bag overnight. For longer storage, freeze the loaf and thaw and reheat just before serving.