Stir sugar and yeast into warm water and wait 10 minutes for foam to form.
Mix in flour to create a paste, cover, and ferment at room temperature for 24 hours.
Prepare second dough by mixing sugar and yeast with warm water until foamy.
Combine flours, salt, yeast mixture, starter dough, and oil in a large bowl.
Mix by hand into a sticky dough, adding a few extra tablespoons of water if dry.
Knead dough on a work surface for 10 minutes until smooth and elastic.
Place dough in an oiled bowl, cover, and rise for 1 hour until doubled in size.
Line a tray with parchment and dust with flour.
Tip dough onto tray, stretch to 30cm, fold in half, and repeat three times.
Shape into a loaf, slash the top, and prove in a warm place for 45–60 minutes.
Preheat the oven to 240°C.
Bake for 20–25 minutes until golden and the base sounds hollow when tapped.
Cool the loaf completely on a wire rack.