Make the Red Cabbage Puré by finely slicing around 1/4 cabbage head, transfer to a small sauce pan and cover with 250/300 grams of water. Bring to boil and for 20-25 minutes or until tender. Allow to cool completely at room temperature and blend to smooth.
Prepare 3 bowls. Measure the water, the starter and salt in one of them. Stir roughly to loosen a bit the sourdough, then add the flours. Combine to form a rough dough. Cover with a cloth and place in a warm area to ferment (between 23 to 26°C - 73 to 79° F). This will be the white/plain one.
Add the starter, the salt and purple cabbage puré to the second bowl. Mix roughly, add the flours and combine once again into a dough. If it feels too dry add a bit of water or more puré 1/2 tbsp at a time.
In a small glass or pot combine the 100 grams of puré with the bicarbonate of soda. Stir well and you'll notice it will turn deep blue. If you want it more intense add a pinch of soda more, but try not to exceed as it will make the bread too bitter.
As per the purple dough stir the salt, the starter and the blue puré first, then mix into the flours (+ some more water, if needed) until you are happy with the dough.
Leave the three doughs to ferment for 30 minutes.
After 30 minutes, grab a corner of the dough and pull it up and into the center. Repeat until you’ve performed this series of folds 4 to 5 times with the dough. Do this for each dough.
Repeat the folding step for 4-5 more times every half an hour or so (It will take 2-3 hours in total).
Lamination. Lightly spread the plain dough into a rectangle. Place on top of it the blue (or purple) dough and spread it roughly to the same size. Repeat the process with the remaining dough.
Carefully (and patiently) start pulling the corners and sides of the stacked doughs to a thin(-ish) sheet, then fold into a letter shape (fold the first side lengthwise to 2/3 of the surface, and overlap the other side on top).
Fold a portion of the dough 3-4 more times over itself from the short side, until you reach the end, then cover with a cloth or bowl and let it rest for an extra 30-40 minutes.
Shape the bread and transfer to the fridge overnight or for at least 12 hours.
To bake. Preheat the oven between 240 and 250°C (470°F). Take the bread out from the fridge seconds before baking it, score and place in a hot Dutch oven.
Cook for 20 minutes with the lid of the Dutch oven on, and another 20-25 minutes uncovered.
Allow the bread to cool for at least 30 minutes before cutting it and...enjoying!