1. Combine Starter and Water: In a large mixing bowl, gently combine the active sourdough starter with the warm water. Use a Danish dough whisk or a sturdy spoon to mix until the starter is mostly dissolved and the mixture looks milky. This ensures even distribution of the yeast and bacteria.

2. Add Flours and Salt: Add the bread flour, whole rye flour, and fine sea salt to the wet mixture. Mix with your hands or a Danish dough whisk until no dry flour remains and a shaggy dough forms. It will be sticky at this stage, which is perfectly normal.

3. Initial Rest (Autolyse): Cover the bowl with a clean kitchen towel or plastic wrap and let the dough rest for 30 minutes. This allows the flours to fully hydrate and gluten development to begin, making the dough easier to work with.

4. Perform Stretch and Folds: Over the next 2-3 hours, perform a series of “stretch and folds.” Every 30 minutes, lightly wet your hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the center. Rotate the bowl and repeat this action 3-4 times until all sides have been stretched and folded. You’ll notice the dough becoming smoother and more elastic with each set.

5. Bulk Fermentation: After the stretch and folds, cover the bowl again and let the dough ferment at room temperature for another 2-3 hours, or until it has visibly increased in volume by about 30-50% and looks bubbly. This is your first rise, also known as bulk fermentation.

6. Shape the Loaf: Gently turn the dough out onto a lightly floured surface. Carefully shape it into a round or oval loaf, creating tension on the surface. Avoid deflating the dough too much. Place the shaped dough seam-side up into a floured banneton or a bowl lined with a floured kitchen towel.

7. Proofing (Second Rise): Cover the banneton or bowl and let the dough proof at room temperature for 2-4 hours, or refrigerate for 8-12 hours for a slower, colder proof. The dough should feel puffy and spring back slowly when gently poked.

8. Bake the Bread: Preheat your oven to 450F (230C) with a Dutch oven inside for at least 30 minutes. Carefully remove the hot Dutch oven, invert the dough into it, score the top with a sharp knife or razor blade, and replace the lid. Bake for 30 minutes with the lid on, then remove the lid and bake for another 10-15 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210F (93-99C).

9. Cool Completely: Transfer the baked bread to a wire rack and let it cool for at least 1 hour before slicing. This is crucial for the crumb to set and for the flavor to fully develop. Slicing too early can result in a gummy texture.