Place sourdough starter (or whey or kefir) into a Thermoserver with the oats and millet.
Cover with water and give the mixture a good stir so the starter is well combined.
Cover and sit at room temperature for 12-24 hours, giving it an occasional stir if you wish.
When ready to eat, tip all the contents into the Thermomix bowl and add salt and vanilla. Top up with a little additional milk or water as some of the liquid has been absorbed. You want it to be quite liquid in the bowl.
Cook 10 min/80°C/Reverse/speed 1 then let it sit in the bowl for another 10 minutes until the liquid has been absorbed.
Divide between serving bowls and go for it with the toppings of choice.