In a medium saucepan, heat the olive oil over medium heat. Add onions and cook until onions are translucent, about 5 minutes. Add the remainder of the ingredients, except the mozzarella cheese. Heat over medium heat, stirring occasionally, until the soup comes to a simmer. Add the mozzarella cheese one handful at a time, stirring to let each addition melt before adding the next. Stir to combine and make sure all the cheese has melted and serve.