1In resealable food-storage plastic bag, mix cheeses and flour. Shake until cheese is coated with flour. In fondue pot, heat broth and milk just to a simmer over warm/simmer setting.
2Add cheese mixture, about 1 cup at a time, stirring with wire whisk until melted. Cook over warm/simmer setting, stirring constantly, until slightly thickened. Stir in tomatoes and onions.
3Keep warm over warm/simmer setting. Serve with breadsticks and vegetables.