In a mixing bowl, combine the active sourdough starter, 20 g of bread flour, 5 g of rye flour, and 25 g of water. Mix until it forms a thick paste. Let this mixture ferment for several hours, or until it becomes bubbly and has doubled in size.

In a large mixing bowl, combine the 900 g of bread flour and 100 g of rye flour. Add 775 g of the water, reserving 25 g for later. Stir until everything is just mixed together and let it rest for approximately 30 minutes.

After the autolyse, add the leaven and the reserved 25 g of water to the flour mixture. Use your hands to mix all the ingredients together until fully incorporated.

Sprinkle the kosher salt over the dough and mix it in thoroughly. Knead the dough slightly in the bowl.

Transfer the dough to a clean, lightly greased bowl. Cover it with a damp cloth or plastic wrap and allow it to rise at room temperature for about 4 to 5 hours, stretching and folding the dough every 30 minutes for the first two hours.

Once the dough has significantly increased in volume, gently turn it out onto a floured surface and shape it into a round loaf.

Let it rest for about 20 minutes to relax.

After resting, flip the dough seam-side up and shape it into its final form. Place it in a well-floured proofing basket and cover it.

Let it proof for 1 to 2 hours, or until expanded. Preheat your oven to 450°F approximately 30 minutes prior. Transfer the loaf onto parchment paper, score the top, and bake in the Dutch oven.

Cover and bake for 30 minutes. Remove the lid and bake for an additional 15 to 20 minutes until golden.

Remove the bread from the Dutch oven and let it cool on a wire rack for at least an hour before slicing.