Cook onion in hot oil in large saucepan over low heat 45 minutes, or until translucent and slightly browned, stirring frequently. Stir in Roasted Garlic Teriyaki Sauce. Add broth and black pepper, bring to a boil. Reduce heat; cover and simmer 10 minutes. Meanwhile, place 1 cheese slice over each toasted bread slice and broil until cheese melts and is golden brown. Divide soup evenly into 6 indivdual soup bowls. Place bread slices on top and serve immediately.