Combine all of the ingredients (300g bread flour, 70g whole wheat flour, 70g whole rye flour, 360g water, 130g active starter, and 8g baking salt) in the bowl of a stand mixer until no dry flour remains.
Cover and let the dough rest for 30 minutes. This hydration period allows gluten to begin developing naturally, making the dough smoother, stronger, and easier to knead.
Mix the dough until it becomes smooth, elastic, and reaches full gluten development. It should easily pass the windowpane test. Depending on your mixer, this usually takes about 6–10 minutes. (Note: A spiral mixer keeps dough cooler, whereas traditional planetary mixers generate more friction heat—keep an eye on dough temperature if using a standard stand mixer).
Lightly shape the mixed dough into a tight ball.
Generously dust your banneton with whole grain flour or fine corn flour to prevent sticking.
Place the dough seam-side up into the banneton. Gently stitch the seams together to create surface tension.
Cover the basket with a clean linen towel. (Do not use plastic, as it traps moisture and can cause the dough to stick).
Allow the dough to proof at a controlled temperature of 26°C (79°F).
Let it ferment until it approximately doubles in size (this takes about 5½ hours in the author's kitchen, but watch the dough rather than the clock). It is ready when a gentle poke with a floured finger leaves an indentation that remains visible or springs back very slowly.
Place the banneton into the refrigerator for at least 2 hours to chill. This firms up the dough, making it much easier to score cleanly and form an ear. (Alternatively, you can leave it in the refrigerator overnight and bake the next morning).
Preheat your oven with a baking steel or stone inside to 500°F (260°C).
Invert the chilled dough onto parchment paper and score the top quickly with a sharp lame.
Transfer the loaf onto the hot baking surface and immediately cover it with a cold baking shell to trap the steam.
Bake for 15 minutes at 500°F (260°C) with the baking shell on.
Carefully remove the baking shell, reduce the oven temperature to 450°F (226°C), and Bake for an additional 20 minutes uncovered.
Transfer to a wire rack and allow the bread to cool completely before slicing.