In a large bowl, mix the bread flour and water until a shaggy dough forms; cover and rest 20–30 minutes for an autolyse.
Sprinkle the fine sea salt and instant yeast over the dough and mix by hand until fully incorporated, 2–3 minutes.
Over the next 60 minutes at room temperature, perform 3 sets of stretch-and-folds every 20 minutes: with damp hands, lift and fold each side of the dough toward the center to build strength.
Cover tightly and refrigerate the dough to cold-ferment for 12–16 hours.
Line a baking sheet with parchment. Generously dust a clean kitchen towel with the all-purpose flour (for dusting) and set it up like a makeshift couche with a few pleats.
Turn the cold dough out onto a clean counter. Using a bench scraper and slightly damp hands, divide into 3 equal pieces and gently pre-shape each into a loose log. Rest, uncovered, 15–20 minutes.
Shape each piece into a baguette: pat into a rough rectangle, fold long edges to the center to tighten, then roll gently from the center outward to 14–16 inches, tapering the ends lightly.
Place the shaped loaves seam side up in the pleats of the floured towel. Cover and proof until noticeably puffy and the dough springs back slowly when pressed, 45–60 minutes.
About 45 minutes before baking, place a baking stone or an inverted baking sheet on the middle rack and a sturdy metal pan on the lower rack; preheat the oven to 475°F/245°C.
Invert each loaf seam side down onto parchment on a peel or backside of a sheet pan. Score each with 3–5 overlapping diagonal slashes at a shallow angle.
Slide the loaves onto the preheated stone (or baking sheet). Pour about 1 cup hot water into the preheated pan to generate steam and close the oven immediately.
Bake until deeply golden with small blisters, 22–28 minutes; the loaves should sound hollow when tapped on the bottom and read 205–210°F inside. Cool on a rack at least 45 minutes before slicing.