Dissolve the salt in the water and pour into a large bowl with the flour.
Crumble in the fresh yeast and start to knead the dough using the dough hooks on the lowest setting, working up to speed 3 for about 7 minutes.
Once the dough pulls away from the side of the bowl, split the dough in two, form balls and cover. Leave in a warm place and let it prove for about 3 hours, or until doubled in size.
Prepare the tomato sauce by adding the oil and oregano to the tomato puree.
Cut the mozzarella into strips and the onion into very thin slices.
After three hours of proving time, the balls will have doubled in size.