Add the flours and 350 g water to the bowl of a stand mixer and mix on low for a couple of minutes, until all the flour is hydrated. Autolyse for 1 hour.
Add the starter, salt, and the last 50 g water and mix on low for 5 minutes.
Cover the bowl with a clean kitchen towel, and let rest for 30 minutes. Stretch and fold 3-4 times in 30 minutes intervals.
Bulk ferment at room temperature for 3-5 hours (depending room temp).
Roughly shape and place (or directly pour) into a greased baking tin. Let the dough rise at room temperature for 2-3 hours.
Bake at 230C with steam for 20 minutes, then again at 220C dry for 30 minutes or until the internal temperature reaches 96-99°C.
