Mix 100g warm water, 100g AP flour, and 45g mature sourdough starter in a bowl
Cover with a dishcloth and let rest overnight for 12-14 hours on the counter
Mix the preferment, 800g warm water, 700g semolina, 300g AP flour, and 20g salt in a big bowl
Allow to rest for 45 to 60 minutes for the flour to absorb the water completely
Toast 150g seeds in a dry pan until fragrant, golden, and lightly popping
Lightly grind the toasted seeds in a mortar and pestle (optional)
Add seeds with the first fold after the initial rest
Do a few extra folds the first time to build gluten structures
Perform bulk rise for five hours after the first fold
Every 45 to 60 minutes, give the dough three or four stretch and folds
Divide and shape the dough, then bench rest for 30 minutes
Shape again and add to a proofing basket
Cover baskets with a cloth and place in the fridge for 12 to 14 hours
Preheat a Dutch oven with the oven at 550°F
Turn out the loaf, score it, and bake with the lid on for 20 minutes
Remove the lid and bake for another 15 minutes or until the centre registers 210°F
Repeat scoring and baking process with the second loaf
