Mix 450ml warm water with 20g yeast and 500g flour in a large container.
Cover and fermented for six to twenty-four hours.
Add 50ml warm water, remaining yeast, remaining flour, mustard, brown sugar, and oil.
Mix until combined and knead on a work surface for five minutes.
Sprinkle over the salt and continue kneading until the dough is elastic.
Divide the dough into two loaves or eight rolls and place on baking trays.
Cover and leave in a warm place to prove for one hour.
Preheat the oven to 230°C and place a deep tray in the bottom.
Place dough in the oven and add 250ml of water to the bottom tray.
Bake for ten minutes and then briefly open the door to release steam.
Bake for fifteen more minutes until the loaves sound hollow when tapped.