Refresh your sourdough starter until it is active and bubbly (1-2 days before). Mix it with the beetroot juice, then add the wholegrain rye, wheat and spelt flours to form a uniform dough.
Cover and let the dough rest for about 30–45 minutes. Add the salt and walnuts (if using), then mix until everything is evenly incorporated.
Cover the dough and leave it to ferment at room temperature for about 10–12 hours, or until it has visibly expanded.
Shape the dough into a loaf, place it into a well-floured banneton or loaf tin, cover, and let it proof for another 10–12 hours.
Preheat the oven to 240°C with a Dutch oven or baking stone. Carefully transfer the dough, score the top, and bake with the lid on (or with steam) for about 20 minutes.
Remove the lid (or steam), reduce the temperature to 220°C if needed, and bake for another 30–35 minutes until the crust is deeply browned.
Allow the bread to cool completely on a wire rack before slicing. This helps the crumb finish setting and gives the best texture.