I posted about this once before, I think. The concept is that each time you ‘feed’ the sourdough starter, you first throw away some of it, so there will be room in the container. 
In this case, it was a specially made starter, made from cooked beetroot (from our garden), flour and water. I let it work for several days before smearing it out on the surface of a silicone pad.
Liberal sprinkling of salt and pepper – and then just put it into the top of the hot water cylinder to dry. That only takes a few days, since it is so thinly spread out.
When dry, it is flakey – and can be blended up to make a powder/flakes that taste great sprinkled on just about anything. Last night, we had it as a topping on poached eggs – which were themselves sitting on fresh slices of sourdough bread!