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 for another 60 mins. Cover and let rest for 30 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, and 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 intil the baguettes appear risen and poofy.
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 in oven. Alternatively, place an oven-safe pan with ice cubes on the bottom rack.
Bake. Bake for 20 minutes with steam, then remove steam source and continue baking for 15 minutes until crust is golden brown.
Cool, slice, + store. Remove from oven and let cool completely on a wire rack before slicing. 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.