Build the Levain (9:00 am):
In a small bowl, mix 46g mature starter, 46g bread flour, 46g water. Let ferment at room temperature for 4–5 hours, until bubbly and active.

Fermentolyse (1:00 pm):
In a large mixing bowl, combine 574g bread flour, 191g whole wheat flour, 551g water (Water 1), and 138g mature levain. Mix until no dry bits remain. Cover and rest for 1 hour.

Add Salt and Water 2 (2:00 pm):
Sprinkle 15g salt over the dough, then pour 61g water (Water 2) on top. Use your hands to squeeze, fold, and incorporate. Mix for 5–10 minutes until the dough tightens slightly.

Bulk Fermentation (3:00–6:00 pm):
Transfer the dough to a fermentation container. Bulk ferment for 3 hours at room temperature, performing two folds.

First Fold (3:30 pm):
Perform a standard coil or stretch-and-fold to begin building dough strength.

Second Fold – Lamination (4:30 pm):
Gently stretch the dough out on a damp surface into a large rectangle. Evenly distribute 153g roasted potatoes, 114g sautéed leeks, and 23g chopped dill (optional). Fold the dough like a letter, then tuck back into a loose round. Return to the container seam-side down.

Divide and Pre-Shape (5:30 pm):
Turn the dough out onto a clean surface and divide into two equal portions (~900g each). Shape into loose rounds and rest for 20–30 minutes.

Final Shape (6:00 pm):
Shape each round into a tight boule or batard. Place seam-side up into a well-floured banneton.

Cold Fermentation (6:45 pm–next morning):
Cover and refrigerate the loaves overnight (12–18 hours) for cold proofing.

Bake (9:00 am):
Preheat your oven and Dutch oven to 500°F (260°C) for 45–60 minutes. Invert one loaf into the Dutch oven, score, and optionally spray with water. Lower oven to 480°F (248°C). Bake 20 minutes covered, then 20–25 minutes uncovered. Cool on a wire rack before slicing.