Heat butter and oil in a pot.
Add sliced onions and stir gently.
Cook slowly until golden and soft.
Sprinkle sugar to boost caramelization.
Add garlic and cook briefly.
Pour in wine and deglaze pan.
Add broths and bay leaves.
Simmer gently for 30 minutes.
Season with salt and pepper.
Toast bread slices until crisp.
Ladle soup into oven-safe bowls.
Top with bread and cheese.
Broil until cheese melts bubbly.