Stir the yeast into the warm water & let proof for 10 minutes. Add the remaining water & the biga, then the flours & the salt. Mix with a wooden spoon for 5 minutes. The dough will not pull away from the side of the bowl. Wet your hands & knead on a lightly floured board to ensure that all the ingredients are well mixed. Turn into a lightly oiled bowl, cover with plastic wrap & let rise at room temperature until it has almost tripled in volume, 3 to 5 hours. This dough works very well if left to rise overnight or in the refrigerator.
Let it come back to room temperature before shaping. Flour your work surface very well & flour a scraper. Turn the dough onto the work surface, but do not punch down. Divide into 3 equal pieces. Work in your filling if using by flattening the dough & sprinkling equal amounts of filing over the dough. Turn in the sides of the dough & roll each piece into a ball, being gentle but at the same time pulling the skin taut. Place the loaves on floured parchment paper set on baking sheets, cover with a heavy cloth & let rise until doubled, about 1 hour. Preheat the oven to 450F. Bake the loaves for 15 minutes. At this point the dough should be set you can remove the paper. Slide the loaves directly onto the baking sheets & bake until crusty & golden, about 30 to 35 minutes.