In a large mixing bowl, combine the warm water, molasses, and active sourdough starter. Whisk until the starter is mostly dissolved.

Add the whole wheat flour, bread flour, and rolled oats to the wet ingredients. Mix with a spoon or your hand until just combined and no dry spots of flour remain. This is a shaggy, rough dough. Cover the bowl and let it rest for 30 minutes (autolyse).

After the autolyse, sprinkle the salt over the dough. If using, pour in the melted butter. Gently fold and knead the dough in the bowl for 2-3 minutes to incorporate the salt and butter. It will feel sticky at first but will come together.

Cover the bowl and let the dough rest for 3-4 hours at room temperature (around 72-75°F / 22-24°C). During this time, perform 3-4 sets of 'stretch and folds' at 30-45 minute intervals.

To perform a stretch and fold: With wet hands, grab a portion of the dough from one side, stretch it upwards, and fold it over to the opposite side of the bowl. Rotate the bowl a quarter turn and repeat this process 3 more times until you've stretched and folded all four sides. This builds strength in the dough.

After the last set of stretch and folds, let the dough rest, covered, until it has increased in volume by about 30-50% and feels airy.

Lightly flour your work surface and gently tip the dough out of the bowl. Gently preshape the dough into a loose round or rectangle and let it rest for 20 minutes, uncovered, to relax the gluten.

While the dough rests, prepare your proofing basket (banneton). Dust it generously with rice flour (or all-purpose flour) to prevent sticking. If using a bowl, line it with a clean kitchen towel and flour it well.

Perform the final shaping: Gently flatten the dough into a rectangle. Fold the bottom third up to the middle, then the top third down over that. Flip the dough over and gently cup it with your hands, rotating and tightening it to form a taut, round or oval loaf. Optionally, wet the top and sprinkle with a handful of rolled oats.

Carefully transfer the shaped dough, seam-side up, into the floured banneton or bowl. Cover with plastic wrap or a shower cap and refrigerate for 12-16 hours (cold proof). This develops flavor and makes the dough easier to handle.

About 45-60 minutes before baking, place your Dutch oven with the lid in the oven and preheat to 450°F (230°C).

Once the oven is fully preheated, carefully remove the hot Dutch oven. Gently invert the cold dough from the banneton onto a piece of parchment paper.

Using a sharp knife or a bread lame, score the top of the dough. A simple straight line down the center or a cross pattern works well.

Carefully lift the parchment paper with the dough and gently lower it into the hot Dutch oven. Place the lid back on.

Bake with the lid on for 25 minutes. After 25 minutes, remove the lid and continue to bake for another 20-25 minutes, or until the crust is deep golden brown and the internal temperature reaches 200-210°F (93-99°C).

Carefully remove the bread from the Dutch oven and transfer it to a wire rack to cool completely for at least 2 hours before slicing. This is crucial for the crumb to set properly.