In a Dutch oven over medium-high heat, melt butter. Add onions; saute until tender. Add sugar; cook and stir until lightly browned. Stir in flour until blended; gradually add the broth, soup and water. Bring to a boil. Reduce heat; cover and simmer for 10 minutes. Stir in Parmesan cheese., Meanwhile, place bread on a baking sheet. Broil 4 in. from the heat for 2 minutes on each side or until toasted. Sprinkle with mozzarella; broil for 2 minutes or until cheese is melted. Ladle soup into bowls; top each with a cheese toast. If desired, top with chopped fresh thyme.