Combine all ingredients. In a large bowl, combine sourdough starter, bread flour, water, and salt. Mix until a shaggy dough forms.
227 g sourdough starter, 400 g bread flour, 237 g water, 6 g kosher salt
Rest for 60 mins. Cover the bowl with plastic wrap and let the dough rest for 60 minutes at room temperature (autolyse period).
Stretch + fold. Turn your dough out onto a floured work surface. Perform a series of stretch and folds: grab one side of the dough, stretch it up, and fold it over itself. Rotate the bowl 90° and repeat one more time.
Rest again, then stretch + fold again. Cover and let rest for 60 minutes, then perform another set of stretch and folds.
Proof overnight in fridge. Cover the bowl with plastic wrap and refrigerate for 12-48 hours. This slow fermentation helps the dough slowly form more gluten and develop a deeper sourdough flavor.
Preheat oven, remove dough from fridge, + divide in half. Preheat oven to 450°F (232°C). Remove dough from refrigerator and turn out onto a lightly floured work surface, then divide the dough into two equal halves.
Shape baguettes. Gently shape each into an elongated loaf, being careful not to deflate too much. Use a gentle stretching motion to lengthen each baguette.
Allow to rise on pan. Transfer shaped loaves to a floured baking sheet or baguette pan. Let rest for 60-90 minutes, or until the baguettes appear risen and poofy. Depending on your starter, the temperature of your kitchen, and how long you let the dough rest in the fridge, it can take up to 2 hours for the loaves to double in size.
Score loaves. Just before baking, score the tops of loaves with a sharp knife, kitchen shears, or bread lame.
Place in oven + add steam. Place bread pan in oven. Now it's time to create a little steam! You can either (carefully!) pour 2 cups of boiling water into a heat-safe baking dish on the rack below the bread, or toss a few ice cubes into the oven when you place your loaves in (my preferred method).
Bake. Bake for 20 -25 minutes, or until deeply golden brown.
Cool, slice, + store. Remove from oven and let cool completely on a wire rack before slicing.
Storing + reheating. Store sliced bread for up to 24 hours at room temperature, or freeze for longer term storage. Unsliced baguettes can be left for 24 hours at room temperature, loosely wrapped in a tea towel or in a paper bag. To reheat, sprinkle water on the outside of the baguette (yes REALLY!) then pop it in the oven to warm + crisp up again.