Build your levain: Feed starter 4-6 hours before you plan to mix, using it at its peak activity.

Prepare Caramelized Onions: In a large skillet over medium-low heat, add the olive oil, sliced onions, and salt. Cook slowly, stirring frequently, for 35-45 minutes until deep golden brown and jammy. Transfer to a bowl to cool completely.

Autolyse: In a large bowl, mix bread flour, all-purpose flour, and 330g warm water by hand until a shaggy dough forms and no dry flour remains. Cover and let rest for 30 minutes.

Final Mix: Add levain to the dough, integrating it by hand. Then sprinkle sea salt over the top and continue mixing until the dough feels cohesive, using the reserved 20g water to help incorporate if needed.

Bulk Fermentation & Folds: Transfer dough to a clean, lightly oiled bowl. Perform four sets of stretch and folds, spaced 30 minutes apart, during the first 2 hours of bulk fermentation.

Laminate inclusions: Spread the cooled caramelized onions and 3/4 of the Gouda cheese evenly over the dough surface, then fold the dough letter-style to incorporate the layers. Transfer back to the bowl.

Complete Bulk Fermentation: Let the dough continue to bulk for another 1.5-2 hours (total 4-5 hours), until volume has increased by 50-70% and the dough feels light and airy.

Pre-Shape: Gently turn the dough onto a lightly floured surface. Perform a loose pre-shape by rounding the dough. Let rest uncovered for 15-20 minutes.

Final Shape & Cold Proof: Perform your final shape (boule or batard). Place the shaped dough, smooth side down, into a well-floured banneton. Cover and cold proof in the refrigerator for 12-16 hours.

Preheat Oven: Preheat oven to 475F (245C) with a Dutch oven inside for at least 45 minutes.

Score & Bake Covered: Turn the cold dough out onto parchment paper. Score decorative pattern. Use the parchment to lift the dough into the preheated Dutch oven. Cover immediately. Bake covered for 20 minutes.

Bake Uncovered: Uncover and bake for another 25-30 minutes, or until the crust is a deep golden brown and the internal temperature reaches 208-210F (98-99C). Let cool completely on a wire rack before slicing.