In a large bowl, combine biga with 350 grams warm water and stir to combine. Add the white and whole wheat flours to the bowl and mix by hand. (The dough will be sticky.)
Cover the bowl with a towel and let rest for 25 minutes or so.
Add salt and 25 grams warm water. Using hands, thoroughly incorporate salt and water into the dough. Cover dough with a cloth and let rise for 30 minutes.
Fold dough by dipping hand in water and folding the dough up from the bottom. Repeat 3 times, turning the bowl one quarter turn each time. Do this every half hour for 2 1/2 hours. The dough should be slightly billowy and have grown 20-30%. If needed, continue to let rise and fold for another hour.
Transfer to a floured work surface and dust top of dough with flour, then flip dough onto work surface, floured side down. Fold one side up onto itself so that the flour on the surface remains totally outside the dough. (This becomes the crust.) Work dough into a taut round. Cover with a towel and let rest 30 minutes.
Line a mixing bowl with a towel and dust generously with flour.
Dust whole wheat flour onto the dough. Use a dough scraper to flip it so that the floured side is down.
Starting with the side closest to you, pull the bottom 2 corners of dough down toward you, then fold them up into the middle third of the dough. Repeat on the right and left sides, pulling the edges out and folding them in over the center. Finally, lift the top corners up and fold down over the previous folds. Roll dough over so that the folded side becomes the bottom of the loaf. Shape into a smooth, taut ball.
Transfer round, seam side up, to prepared bowl. Cover with a towel and let rise 3 to 4 hours, OR let dough rise in the fridge for up to 12 hours. Allow to come to room temp before baking.
When ready to bake, place a Dutch oven and lid into the oven and heat to 500 degrees. Dust top of dough with whole wheat flour. When hot, remove Dutch oven and lid. Score the top of the loaf a few times and gently place dough into the hot pot, then into the oven. Reduce heat to 450 degrees and cook for20 minutes. Carefully remove lid and bake for another 20 minutes, or until crust is a rich, golden brown.
Transfer to a wire rack to cool. Wait at least 15 minutes before slicing.