Place levain, water, flour and salt into the Thermomix bowl and blend 6 sec/speed 6.
Drizzle dough with EVOO and cover with lid. Allow to sit in the bowl for 1 hour.
Knead 2 min/Interval/dough setting. Tip dough out into a square or rectangular container with a lid. Leave on your kitchen bench for up to 6 hours. Occasionally do a pull and fold. Up to 3 times. Place container into the fridge for up to 18 hours.
When ready to bake, preheat oven to 250°C for a minimum of 30 minutes. Line a cast iron flat tray or pan with baking paper.
Dust the bench liberally with flour and tip the container with the dough upside down, allowing the dough to eventually release and fall out of the container. Use a scraper to divide the dough into two. Dust the top with flour and add a few salt flakes if desired. Transfer the dough onto the prepared tray as two loaves. Throw a cup full of ice into the bottom of the oven before placing the tray inside. Bake 20 minutes. Reduce heat to 200°C and bake a further 30 minutes.
Baking time with sourdough is always a lot longer with sourdough than yeast risen doughs. You need to learn the intricacies of your oven, which can really impact the end cooking time.