Make the leaven: In a jar, mix 50g active starter, 20g bread flour, 5g rye flour, and 25g water. Cover loosely and let rise in a warm place until tripled and bubbly, about 1–3 hours.
Autolyse the dough: In a large bowl, mix 900g bread flour, 100g rye flour, and 750g water until no dry flour remains. Cover and rest while the leaven becomes active, about 1–3 hours.
Add leaven: Add the active leaven to the dough and mix by hand until fully incorporated, about 5 minutes. Cover and rest 30–60 minutes.
Mix salt water: Dissolve salt in 50g hot water. Let cool.
Add salt: Pour salt water over the dough. Mix by hand until fully incorporated, about 5 minutes. Cover and rest 15 minutes.
Coil folds: Perform 4 sets of coil folds at 15-minute intervals over 1 hour. Cover between each set.
Bulk fermentation: Cover and let the dough rise at room temperature until doubled, soft, and bubbly, about 8–12 hours.
Preshape: Turn dough onto a work surface and divide into 2 pieces. Gently shape each into a round. Rest uncovered for 30 minutes.
Shape: Shape each dough into a tight round and place seam-side up into floured bannetons or bowls lined with towels.
Cold proof: Cover and refrigerate for 8–24 hours.
Preheat oven: Preheat oven to 500°F (260°C) with Dutch ovens inside for at least 30 minutes.
Score: Turn dough onto parchment, remove towel, and score with a sharp blade.
Bake covered: Transfer to hot Dutch ovens, cover, and bake for 35 minutes.
Bake uncovered: Remove lids, reduce heat to 450°F (230°C), and bake 15 minutes more, until deep golden brown.
Cool: Cool on a rack for at least 1 hour before slicing.