Heat the oil and butter in a non-stick frying pan. Reduce the heat, add the onions and salt, cover and cook over a medium heat for approx. 20 mins., stirring occasionally. Remove the lid, sprinkle with sugar, continue to cook for approx. 10 mins. until the onions turn light brown. Remove the onions, leave to cool.
Mix the flour, salt and yeast in a bowl. Pour in the water, mix thoroughly with a wooden spoon. Mix in the onions. Cover the dough and leave to rise at room temperature for approx. 16 hrs.
Place the dough on a lightly floured work surface, shape into a ball. Place the dough ball on a well-floured tea towel, sprinkle with a little flour, cover and leave to rise at room temperature for a further 2 hrs.
Place the cooking pot (incl. lid) in the lower half of the oven. Preheat the oven to 240°C. Place the dough ball on a sheet of baking paper (approx. 20 cm in diameter), place in the pot along with the baking paper, cover and bake for approx. 30 mins. Remove the lid and cook for a further 15 mins. Take the pot out of the oven, remove the bread from the pot, leave to cool on a rack.