Mix bread flour, whole-wheat flour, water, and yeast until evenly incorporated. Should be stiff but a small amount of additional water may be necessary (for what, you might ask; couldn't tell you cuz it doesn't say).
Place in bowl, cover, and leave at about 70° for 12-16 hours.
Make the dough
Mix bread flour, whole-wheat flour, yeast, water, and salt until evenly incorporated ("3 min in a spiral mixer on first speed").
Continue mixing and add biga in small chunks. Add additional flour or water as necessary.
Mix until "medium consistency, but with perceptible dough strength and gluten development."
Add walnuts and raisins and mix on first speed until evenly incorporated.
1st rise: 1 hour.
Fold.
2nd rise: 1 hour.
Preshape and rest for 10-20 min (or divide and shape for rolls).