Whisk the active sourdough starter into lukewarm water until dissolved. Add bread flour and mix until you have a shaggy, sticky mass.
Cover the bowl and let the dough rest at room temperature for 30 minutes to 1 hour.
Sprinkle the salt over the dough. Wet your hands, then perform 4-6 stretch and folds by grabbing one edge, stretching it up, and folding it toward the center.
Cover the bowl and rest for 30 minutes. Perform another set of stretch and folds. Repeat for a third set. Then bulk ferment for 6-8 hours until doubled in size with bubbles on the surface.
Turn the dough onto a lightly floured surface. Pull the edges toward the center to create tension, then flip and form into a round ball. Rest uncovered for 20-30 minutes.
Dust your banneton with flour. Flip the dough, fold the bottom third up and top third down, then roll tightly from top to bottom. Pinch the seam closed and place seam-side up in the banneton.
Cover the banneton and refrigerate for at least 12 hours, preferably 24-48 hours for better flavor.
Place your Dutch oven with lid on in a cold oven. Preheat to 500°F (260°C) for a full hour.
Turn the dough out onto a parchment paper. Dust with flour if desired. Make one deep score across the top with a bread lame at a 45-degree angle. Transfer dough to the hot Dutch oven using the parchment. Cover and bake at 500°F for 20 minutes. Remove lid, reduce heat to 450°F (230°C), and bake for 25-30 minutes more until deep golden brown.
Lift the bread onto a wire cooling rack using the parchment. Cool completely for at least 1-2 hours before slicing.