Mix the active starter and lukewarm water in a large bowl. Add the bread flour and fine sea salt, then stir until no dry flour remains.

Cover the dough and let it rest for 30 minutes. This short rest gives the flour time to absorb the water and makes the dough easier to handle.

Over the next 1-2 hours, complete 3-4 sets of stretch-and-folds. Pull one side upward, fold it over the center, rotate the bowl, and repeat on all four sides.

Cover the bowl and bulk-ferment the dough at room temperature until it looks puffy, has risen roughly 50-75%, and shows bubbles. Depending on room temperature and starter activity, this may take 6-12 hours.

Turn the dough onto a lightly floured surface and shape it into a tight round. Place it on parchment and let it rest for 30-90 minutes while the slow cooker heats, or refrigerate it for a longer final proof.

Place the shaped dough, still on the parchment, into the slow-cooker crock and score the top with a sharp knife or bread lame. Cover with the lid and cook on High for approximately 2-2 1/2 hours without adding water.

Check that the center reaches about 200-205°F (93-96°C). Lift the loaf out using the parchment, transfer it to a rack, and cool completely for at least 1 hour before slicing.