Preheat oven to 350°. Place the soup in a large saucepan; bring to a boil. Reduce heat to low; cover and keep warm.
While the soup is heating, top each toasted bread round with 1 cheese slice; place on a baking pan. Bake in middle of oven until cheese is melted (about 5 minutes).
Add the sour cream and basil to soup. Ladle soup into bowls; top with toasts. Sprinkle with pepper and garnish with basil, if desired. Serve immediately.