The evening before baking, stir the 150 g flour, 150 g water and 0.2 g yeast together until no dry flour remains. Cover with room for expansion. Leave 10–14 hours around 68–72°F (20–22°C), until bubbly and aerated, with the domed top just beginning to flatten. Temperature changes the timing.

Stir the poolish with the remaining 200 g water. Add 350 g flour, 3 g yeast and 10 g salt. Mix until all flour is moistened. Cover and rest 20 minutes, then knead gently for 3–5 minutes until more cohesive. The dough should remain soft and tacky; do not add flour just to make it dry.

Cover and let rise about 2 hours. After 30 minutes and again after 60 minutes, use damp hands to lift and fold each of four sides toward the center. Keep covered between folds. At the end, look for a puffier, aerated dough about 50–75% larger; allow more time in a cool kitchen.

Turn onto a lightly floured counter. Divide into 3 equal pieces, about 288 g each before dusting flour. Gently form short logs, cover and rest 20 minutes.

Gently pat one piece into a rectangle without pressing out all the gas. Fold the upper third toward the middle and seal; fold the lower third toward the middle and seal. Fold lengthwise in half and pinch the seam closed. Roll gently from the center outward to a roughly 14-inch loaf. If the dough springs back, cover and rest 5–10 minutes before continuing. Repeat.

Set seam-side down on an 18 × 13-inch parchment-lined baking sheet, leaving about 2 inches between loaves. Cover loosely and proof 45–60 minutes, until puffy and a gentle fingertip indentation returns slowly. During proofing, preheat a conventional oven to 450°F (230°C), with the bread rack in the center. For optional steam, put a heavy uncoated metal or cast-iron pan on a lower rack only if your oven manual permits it. Do not put it on the oven floor or use glass or ceramic.

Uncover. With a sharp lame angled about 30–45 degrees to the surface, make four shallow, slightly overlapping diagonal cuts along each loaf, about 1/4 inch deep. Put the bread tray on the center rack. If using steam, wear dry mitts and carefully pour 120 ml boiling water into the lower hot metal pan using a long-spouted heatproof vessel. Stand back, keep your face clear, avoid splashing the oven glass or light, and close the door. Omit steam if the oven manufacturer prohibits it.

Bake 25–30 minutes, until richly golden brown. After about 15 minutes, briefly open the door while standing back to release steam, then close it and finish baking. Leave the steam pan in place until completely cool; do not carry it out hot. Move the loaves to a wire rack and cool at least 30 minutes before slicing.