Build the Levain - The night before you wish to mix the dough, combine 1 tablespoon mature starter, 100 grams water and 100 grams all-purpose unbleached flour in a clean jar. Cover and let sit overnight.
Autolyse -Mix the levain, 800 grams bread flour, 200 grams whole wheat flour and 700 grams of water in a large mixing bowl with your hands or a spatula until it forms a sticky, shaggy mass. Cover the bowl with plastic wrap or a towel and let rest for 1 hour.
Add Salt - Add 20 grams of salt and the remaining 50 grams of waterto the dough. Use your hands to pinch the dough so that the salt and water are incorporated well. Cover the bowl and let rest for 1 hour.
Bulk Fermentation -Remove the cover and stretch and fold the dough. Starting on the right side, wet your hand and then pull the dough up and over it's self to the opposite side of the bowl. Turn the bowl a quarter-turn and repeat this 3 times until you have completed 4 stretch and folds. Repeat for a total of 3 sets every 30 minutes. Cover and let rise on the counter at room temperature for 4-5 hours or until the dough has risen 1.5 times it's original size.
Pre-shape - Turn the dough out onto a clean surface and cut the dough in half. Use the bench scraper to shape the dough into a tight round. Cover with a towel and let rest for 20 minutes.
Final Shape - Lightly flour the top of the dough and flip it over using the bench scraper. Pull the right side up and over to the center. Repeat with the left, top and bottom sides. Flip the seam side down, cup your hands around the dough and gently pull it towards your self. Turn the dough in a circular motion on the work surface and repeat these steps until the dough has a "tight skin". Dust the top of the dough with flour and place it in a bowl, seam side up, that's been lined with a floured tea towel. Let rest on the counter for 30 minutes, cover with plastic and place in the refrigerator overnight to ferment.
Bake - Preheat oven to 450 degrees (without the Dutch oven inside). Remove the sourdough from the refrigerator, place a piece of parchment paper on top and invert the dough onto the paper. Use the paper to pick up the dough and place it into the cold Dutch oven. Score the top of the dough with a razor or sharp knife. Cover and bake for 30 minutes. Remove the cover and bake for 15-20 minutes.
Cool -Carefully remove the bread from the Dutch oven and let cool for 2 hours before slicing.