Mix the active starter with the water using a spatula until it's soapy in texture.

Add the flour and knead the dough with a stand mixer's dough hook for a couple of minutes. Let it rest for 20 minutes for autolyse.

Add the salt and flax seeds and knead with the same hook for another 5 minutes.

Shape the dough into a ball and leave it in a warm place for a couple of hours to ferment, giving it a stretch and fold during that time.

Refrigerate the dough ball for about 8 hours. In the morning, score it with a disposable blade and preheat the oven to 250 °C.

Place a baking sheet on the rack below. Put the bread in the oven, pour half a cup of boiling water into the pan below and bake for 5 minutes.

Open the oven, mist the bread and the oven walls well with water to create steam, close the door and bake for another 10 minutes.

Lower the temperature to 220-230 °C and finish baking for another 15-25 minutes.

Let the finished bread cool completely before slicing.