In a large mixing bowl, combine bread flour, active sourdough starter, and room temperature beer. Mix until the ingredients come together, forming a shaggy dough. Cover the bowl with a damp cloth and let it rest for 30 minutes to allow the flour to hydrate.

After the autolyse, sprinkle fine sea salt over the dough. Use your hands to incorporate the salt into the mixture, ensuring it's fully integrated.

Place the dough in a lightly greased bowl, covering it with a damp cloth. Allow the dough to rise at room temperature, performing 4 gentle stretch and folds at 30-minute intervals for about 3 to 5 hours.

Turn the dough out onto a lightly wet work surface. Spread it into a rectangle, distribute the cubed Gouda cheese and caramelized onions evenly over the surface, then fold the dough over the fillings.

Form a loose round shape with the dough. Let it rest for 20 to 30 minutes covered with a kitchen towel.

Shape the dough into a tighter boule or batard, then place it seam-side up in a floured banneton. Cover and refrigerate overnight.

Preheat your oven to 475°F (245°C). Transfer the loaf into a preheated Dutch oven, cover, and bake for 20 minutes, then uncover and bake for another 20 minutes until golden brown.

Cool on a wire rack for at least 2 hours before slicing.