In a small jar or bowl, combine 30g active sourdough starter, 30g warm water (around 90-95°F / 32-35°C), and 30g bread flour. Mix until smooth, cover loosely, and let it ferment at room temperature (68-75°F / 20-24°C) for 8-12 hours, or until it has doubled in size, is very bubbly, and passes the float test. This is typically done the evening before baking day.

In a small heatproof bowl, combine 50g rolled oats (not instant) and 100g boiling water. Stir well, cover, and let it sit for at least 30 minutes, or overnight in the refrigerator, until all the water is absorbed and oats are soft. This can be done the evening before or morning of baking day.

In a large mixing bowl, combine 400g bread flour, 50g whole wheat flour, and 270g warm water. Mix with your hands or a dough scraper until no dry flour remains. Cover the bowl and let it rest for 30-60 minutes. This process, called autolyse, allows the flour to fully hydrate and gluten to begin developing. This step is typically done on the morning of baking day.

Add the prepared leaven (approx. 90g), the prepared soaker (approx. 150g), and 10g fine sea salt to the autolysed dough. Mix thoroughly, kneading by hand in the bowl for 5-7 minutes until all ingredients are well incorporated and the dough starts to develop some elasticity. The dough will be slightly sticky due to the oats.

Cover the bowl and let the dough rest for 30 minutes. Perform your first set of stretch and folds: gently stretch a portion of the dough up and fold it over itself. Rotate the bowl and repeat 3-4 times until all sides have been folded. This helps build strength in the dough.

Repeat the stretch and fold process every 30-45 minutes for a total of 3-4 sets over 2-3 hours. The dough should become smoother, more elastic, and increase in volume by about 20-30%. The exact timing depends on your ambient temperature.

While the dough is undergoing bulk fermentation (after the first couple of folds), prepare the filling. Melt 1 tbsp unsalted butter in a small saucepan over medium heat. Add 1 medium apple (peeled, cored, and finely diced) and cook for 3-5 minutes until slightly softened but still holding its shape. Stir in 2 tbsp brown sugar (packed), 1 tsp ground cinnamon, 1/4 tsp ground nutmeg (optional), and 1 tbsp lemon juice. Cook for another 1-2 minutes until the sugar has dissolved and created a light syrup. Remove from heat and let cool completely before adding to the dough.

During your last set of stretch and folds (Step 6), gently spread the cooled apple cinnamon filling over the dough before folding it. Ensure the filling is evenly distributed as you fold the dough over itself.

Lightly flour your work surface. Gently turn the dough out onto the surface. Preshape the dough into a loose round or rectangle. Cover with a kitchen towel and let it rest for 20-30 minutes. This helps the dough relax before final shaping.

Gently flatten the dough into a rough rectangle. Fold the top third of the dough down to the center, then fold the bottom third up over the top. Press down firmly to seal. Then, fold one side over about two-thirds of the way, then the other side over that, creating a tighter log. Roll the dough slightly to create tension and a smooth surface. Place the shaped dough seam-side up into a well-floured loaf pan (9x5 inch) or a proofing basket that fits the loaf pan dimensions.

Cover the loaf pan or proofing basket with plastic wrap or a reusable shower cap. Refrigerate for 12-18 hours (or up to 24 hours). This slow cold proof develops flavor, improves texture, and makes the dough easier to handle for baking.

The next day, preheat your oven to 450°F (232°C) with a Dutch oven (if using) inside for at least 30 minutes. If using a regular loaf pan, preheat the oven with a baking sheet, and place a separate oven-safe pan with 1 cup of water on the bottom rack for steam generation.

Carefully remove the loaf pan/proofing basket from the fridge. If using a Dutch oven, gently invert the dough from the proofing basket into the hot Dutch oven. Score the top of the loaf with a sharp knife or razor blade (lame). If using a loaf pan, place it on the preheated baking sheet. Bake covered (if using Dutch oven) or with steam (if using loaf pan) for 20 minutes.

Reduce oven temperature to 425°F (218°C). Remove the lid from the Dutch oven or carefully remove the steam pan from the oven. Continue baking for another 25-30 minutes, or until the crust is deeply golden brown and the internal temperature reaches 200-210°F (93-99°C).

Carefully remove the loaf from the oven and transfer it to a wire rack to cool completely for at least 2-3 hours before slicing. Slicing too early will result in a gummy texture and loss of moisture.