Add water and sourdough starter to the Thermomix bowl. Mix 5 sec / speed 4.
Add rye flour and salt (and vital gluten if using). Mix 10 sec / speed 4, scrape down, then mix 5 sec / speed 4.
The mixture should be a thick, sticky paste rather than a dough.
Bulk Ferment
Leave in the bowl or transfer to a container and cover loosely.
Allow to ferment for 3–5 hours in a warm place (around 22–26°C), until slightly risen with small cracks forming on the surface.
Shape
With wet hands or spatula, transfer the mixture into a loaf tin. Smooth the top.
Optional: Lightly dock the surface with a wet finger or skewer. (giving somewhere to release pressure)
Final Proof
Leave to prove for 1–2 hours, until slightly puffed with fine cracks on the surface. Do not wait for a large rise.
Bake
Preheat oven to 220°C.
Bake for 15 minutes at 220°C, then reduce to 180°C and bake for a further 45–55 minutes.
Optional: Place a small dish of water in the oven for steam.