Combine bread flour, active sourdough starter, and beer until shaggy dough forms. Let rest for 30 minutes.

Sprinkle salt over the mixture and knead until smooth.

Cover and let dough undergo bulk fermentation for 3-5 hours with stretch and folds every 30 minutes.

Turn dough out and stretch into a rectangle. Add onions and Gouda, fold like a letter.

Shape into a loose round and let rest covered for 20-30 minutes.

Shape into a tight boule and place in a floured banneton.

Refrigerate covered overnight for cold fermentation.

Preheat oven to 475°F (245°C). Transfer dough to parchment and score the top. Bake covered for 20 minutes, then uncovered for another 20 minutes.

Transfer loaf to a wire rack to cool for at least 2 hours before slicing.