Combine the biga, water, and 30 grams of the flour in a deep bowl. Stir until it forms a scraggly-looking dough.
Sprinkle 20 grams of flour on a counter or board. Scrape the dough onto the counter. Sprinkle remaining flour on top. Knead until it forms a smooth, stiff dough, 2 to 3 minutes.
Shape dough, and set in a floured proofing basket. Dust top with flour, cover with plastic film (a disposable shower cap is perfect, too, and reusable). Let sit in a warm spot until doubled in volume, 3 to 4 hours.
minutes before ready to bake, add a cloche or dutch oven to the center rack of the oven. Preheat to 500ºF.
Gently invert the proved loaf onto a sheet of parchment, taking care not to plop it down, or you’ll smack out all the air that’s incorporated. Slash with a sharp knife, as desired.
Transfer to the base of the cloche, cover, and bake 30 minutes, until golden, Remove cover, and bake 10 minutes more until deeply golden. Best to let the bread cool completely before slicing, about 2 hours.