In a medium bowl, combine 1 cup (125g) bread flour, ½ cup (120ml) room-temperature water, and ¼ teaspoon (1g) instant yeast. Stir until no dry streaks remain and a rough, shaggy mass forms.
Cover the bowl tightly with plastic wrap and leave at room temperature for at least 12 hours and up to 24 hours, until bubbly and fragrant.
The next day, add the final dough ingredients and the prepared biga to your stand mixer bowl. Mix on low speed for 2 minutes, then increase to medium speed and knead for 8 to 10 minutes until glossy and elastic.
Lightly oil a large bowl, scrape the dough into it, cover, and let rest for 30 minutes.
Perform a stretch and fold: wet your hand, stretch one side of the dough up and fold it over the top. Rotate the bowl and repeat three more times. Cover and rest another 30 minutes. Repeat this process three more times, for a total of four sets over 2 hours.
After the final stretch and fold, cover the bowl and let the dough rest undisturbed for an additional 30 to 60 minutes until airy, jiggly, and full of bubbles.
Generously flour your work surface and use a bench scraper to gently coax the dough out of the bowl. Divide the dough into two equal pieces, each roughly 400 to 450 grams.
Dust the tops of both pieces heavily with flour. Gently stretch each piece into a rectangle, roughly 10 x 4 inches, keeping as many air bubbles intact as possible.
Line a sheet pan with parchment paper and dust generously with flour. Carefully transfer the shaped loaves to the pan, spacing them at least 3 inches apart.
Dust the tops of the loaves generously with flour, cover loosely, and let proof at room temperature for 45 to 60 minutes until puffed and airy.
While the loaves proof, place your baking stone on the middle rack with a broiler pan or cast iron skillet below. Preheat the oven to 450°F (230°C) for at least 30 minutes.
Slide the parchment paper with both loaves onto the preheated baking stone. Pour a cup of hot water into the broiler pan or skillet to create steam and close the oven door immediately.
Bake at 450°F (230°C) for 20 to 25 minutes, rotating the pan once halfway, until deep golden brown and hollow-sounding when tapped.
Transfer the finished loaves to a wire cooling rack and cool completely for at least 30 to 45 minutes before slicing.