Add about 1/4 of the hot tomato broth. Cook, stirring regularly, until most of the liquid has evaporated, then add another 1/4 of the broth. Continue adding the broth 1/4 at a time, regularly stirring while the rice absorbs the liquid. The rice will begin to swell and become somewhat creamy in texture. The process will take 20-25 minutes. Add mozzarella and stir until melted. Add salt and pepper to taste.