Mix the sourdough starter with 100 g water and 100 g all-purpose flour until smooth. Cover and rest at room temperature until bubbly, about 4–6 hours.
In a large bowl, combine bread flour, oats, and salt. Stir in the levain, lukewarm water, and honey until a shaggy dough forms. Let rest for 20–30 minutes.
Perform stretch-and-folds on the dough every 20–30 minutes for about 2 hours until it becomes stronger and smoother.
Cover the dough and allow it to bulk ferment for 3–4 hours until noticeably expanded.
Turn the dough onto a floured surface, shape into a round boule or bâtard, and place seam-side up in a floured banneton or bowl lined with a towel.
Proof the dough covered at room temperature for 1–2 hours until slightly puffy, or refrigerate overnight for deeper flavor.
Preheat oven to 230 °C with a Dutch oven or baking stone inside. Carefully invert the loaf onto parchment, score the top, and place into the hot Dutch oven.
Bake covered for 20 minutes, then remove the lid and bake 15–20 minutes more until golden and hollow sounding when tapped.
Cool the bread completely on a wire rack before slicing to set the crumb.