Place all ingredients into the Thermomix bowl and knead 3 min/Interval, dough setting. Tip out into a bowl, cover and leave to proof for 24 hours with a couple of pull and folds as described in the lazy sourdough loaf video. Keep the folds underneath the dough as it rises.
When the dough has risen sufficiently to your liking, tip out onto a floured bench and do the last pull and folds needed. Form the loaf and rise it in a besan dusted banneton while you heat the Dutch oven to as high a temperature as it can go. (I go to 275°C) Rise at least a further 30 minutes.
Tip the bread onto a piece of baking paper, dust well with the besan flour, and then cut as you wish with a lame.
Transfer the paper and loaf to the hot Dutch oven, cover and bake 25 minutes on this high temperature. Remove cover, reduce temperature to 200-200°C and bake a further 20 minutes. The loaf will be dark and sound hollow when tapped.
Cool on a wire rack before cutting. Serve as you would any other sourdough, though this loaf is delicious with salmon, cheeses, fresh butter, salad, shaved ham etc.