In the morning, prepare your leaven: Mix your sourdough starter with lukewarm water and flour and let it rest in a warm spot (around 28 degrees Celsius) for 4 hours. It's ready to use when it has doubled or tripled in size, looks bubbly and still has a slightly domed top.
After mixing the leaven, start the autolyse: In a mixing bowl, combine the whole grain flour and warm water. Make sure that there are no dry patches of flour left. Cover with a tea towel and let it sit on your counter until the leaven is ready to use.
Prepare the main dough: When the leaven is ready, add it to the autolyse dough and give it a quick knead, to bring everything together. Add the salt and dimple it into the dough with your fingers. Then, perform a few rounds of the slap & fold technique to build dough strength, about 2-5 minutes.
For the bulk fermentation, place the dough in a lightly oiled mixing bowl, cover with a damp tea towel and let it rise in a warm spot (28 degrees Celsius). Perform a round of coil folds in 45-60 minute intervals to build strength and elasticity. After 3-3.5 hours, you should see visible signs of fermentation. At this stage, the dough should have grown in size. You should see lots of smaller air bubbles throughout the dough and some larger ones on the surface. If the dough still looks dense, let it rest longer.
Flour the banneton and set aside. Time to pre-shape: Gently turn the dough out onto a lightly floured surface, bring all 4 corners of the dough into the center, overlapping, to form a loose ball. Flip it over and let it rest for 20-30 minutes. Flip the dough over again and shape a boule. Transfer the dough to the banneton, cover it with a plastic bag and put it in the fridge overnight.
The next morning, pre-heat the oven to 230 degrees Celsius with the Dutch oven inside. Turn the dough out on a piece of parchment paper, score, and bake for 30 minutes inside the dutch-oven. Then, gently lift the bread out of the Dutch oven and finish baking directly on the oven rack for another 10 minutes, or until the crust has a deep brown color. Let the loaf cool down completely before slicing. If you cut into it while it's still hot, you risk ending up with a gummy and sticky crumb.