Heat oil and butter in a large nonstick skillet over medium heat until butter melts. Add onion; cook, uncovered, 40 minutes or until golden brown, stirring occasionally. Add broth, wine, and pepper. Bring to a boil; reduce heat, and simmer, uncovered, 15 minutes.
Ladle 1 cup soup into 4 ovenproof bowls. Place 1 bread slice in each bowl; sprinkle evenly with cheese. Place soup bowls on a large baking sheet, and broil 1 minute or until cheese melts. Serve immediately.