Pour 2 cups lukewarm water into a large bowl or the bowl of a stand mixer. Add sourdough starter and whisk vigorously to combine.
Add the yeast, whole wheat flour and 1 ½ cups of the bread flour and stir until smooth.
Add the remaining cup of water and salt. Mix thoroughly.
Add the remaining bread flour and stir until combined. The dough will be wet and sticky.
Cover bowl with plastic or a lid (if mixed in a stand mixer, transfer to large bowl). Let rise for 12-15 hours.
After dough has risen, punch it down, then sprinkle generously with flour (about 1 cup) and "turn" the dough. Lift one side of the dough and fold it into the center. Repeat on the opposite side, then rotate the bowl and fold in each perpendicular side for a total of 10-12 folds, until the extra flour is worked in. Keep your hands well floured during this process to avoid a sticky mess.
Transfer dough onto a large sheet of parchment paper. Clean bowl or get a new large bowl.
Transfer dough the parchment paper along with the dough into the bowl and let rise again for about 2 hours, until bread has reached desired height .
When there is 30 minutes left in the rise time, place a dutch oven in the oven and preheat to 450°F for 30 minutes (see note).
Picking the parchment paper up by the corners, carefully transfer the dough and parchment paper into the hot dutch oven. Do this very gently as you don't want the dough to deflate, and carefully to avoid burns on the hot pot.Cover and bake for 30 minutes (see note).
Remove lid. Bake for an additional 15 minutes. Remove from dutch oven, peel off the parchment (it will be brittle and break easily) and cool on a wire rack.