Feed your sourdough starter 8-12 hours before mixing the dough. Combine 50g starter, 100g bread flour, and 100g water in a jar. Stir well, cover, and let it sit at room temperature.
In a large bowl, mix 500g bread flour and 350g water until no dry flour remains. Cover and let it rest for 30-60 minutes.
Add 100g active starter and 10g salt to the autolyse mixture. Mix until fully incorporated.
Perform stretch and folds every 30 minutes for 2 hours. Cover the bowl and let it ferment for 4-6 hours at room temperature, until it has doubled in size.
Gently shape the dough into a round or oval loaf. Place it seam-side up in a floured proofing basket or bowl. Cover and refrigerate overnight (8-12 hours).
Preheat your oven to 475°F (245°C) with a Dutch oven inside for 30 minutes.
Carefully transfer the proofed dough into the hot Dutch oven. Score the top with a lame or sharp knife. Bake with the lid on for 20 minutes, then remove the lid and bake for an additional 20-25 minutes, until the crust is deep golden brown.
Let the bread cool on a wire rack for at least 30 minutes before slicing.
Leave 200g unsalted butter at room temperature until softened, about 1-2 hours.
Using a stand mixer or hand mixer, whip the butter on medium-high speed for 5-7 minutes, until light and airy. Add 1 tsp sea salt flakes and 1 tbsp chopped fresh rosemary (optional). Mix until combined.
Transfer the whipped butter to a bowl, cover, and refrigerate until ready to serve.