100 g active sourdough starter (1/2 cup), fed 4 to 8 hours before and bubbly
375 g water at about 27C/80F (1 2/3 cups), divided
450 g bread flour (3 2/3 cups)
50 g whole wheat flour (1/3 cup + 1 tbsp)
10 g fine salt (1 3/4 tsp)
Rice flour, for the basket
How to make it
Mix (5 min plus 1 hr rest): In a big bowl, stir the starter into 350 g of the water. Add both flours and mix by hand until no dry flour is left. Cover and rest 1 hour (autolyse).
Add salt (5 min): Sprinkle the salt and the last 25 g water over the dough and squeeze it through with wet fingers until absorbed.
Bulk ferment with folds (4 to 5 hr): Keep the dough somewhere warm, about 24 to 26C (75 to 79F). Every 30 minutes for the first 2 hours, do a set of folds: with a wet hand, lift one side of the dough up and over, turn the bowl a quarter, and repeat four times. Then leave it alone. It's ready when it has grown about 50%, feels airy, jiggles, and has bubbles on the sides.
Pre-shape (5 min plus 20 min): Tip onto an unfloured counter and use a bench scraper to push it into a loose round. Rest uncovered 20 minutes.
Shape (5 min): Flour the top, flip it, and fold the bottom third up, the sides in, and the top down. Roll it toward you to build tension on the surface. Set it seam-side up in a basket or towel-lined bowl dusted with rice flour.
Cold proof (12 to 16 hr): Cover and refrigerate overnight. The cold builds flavor and makes it easier to score.
Bake (45 min): Put a Dutch oven in the oven and heat to 250C (480F) for 45 minutes. Turn the cold dough out onto parchment, score one long, confident cut 1 cm deep at a shallow angle, and lower it in. Lid on, bake 20 minutes. Lid off, lower to 230C (450F) and bake 20 to 25 minutes more, until the crust is deep brown and the loaf sounds hollow underneath (internal 96C/205F).
Cool (2 hr): Cool on a rack at least 2 hours before cutting; the crumb is still setting.
Tips
Watch the dough, not the clock: a cold kitchen can double bulk time.
Too dense: under-fermented. Flat and sour: over-fermented.
A rye loaf: swap 150 g of the bread flour for whole rye and add 25 g more water.