For the biga, stir the water, bread flour, whole wheat flour and yeast together in a bowl with a wooden spoon until evenly blended. Cover and set this starter aside on the counter for 12 to 18 hours until soft and bubbly.

For the dough, stir the biga and water with a spoon or hands in a large bowl so that the biga breaks down a little and becomes softer. Add the flour and yeast and mix by hand with a wooden spoon. Let the dough sit uncovered for 15 minutes, then add the salt and stir in vigorously until the dough feels stretchy.

Cover the bowl with plastic wrap and let sit on the counter for 2 hours until doubled in size.

Line two baking trays with parchment paper and dust them with flour. Turn the dough out onto a well-floured work surface. Using your hands and/or a bench scraper with lots of flour, divide the dough in half and shape each piece into a rough 8 × 12-inch rectangle about 1½ inches tall. Transfer each piece to a baking tray and let rest for 30 minutes uncovered.

Preheat the oven to 450°F (230°C). Dust the tops of the loaves with flour and bake for about 30 minutes until even golden brown. Turn off the oven and leave the bread in for 10 minutes to set the crust.

Transfer the loaves to a cooling rack for at least 15 minutes before slicing. Best enjoyed the day it is baked, but great for toasting or grilling the next day, or can be frozen.