Caramelize Onions: In a pot, cook onions in butter & oil over low heat for 30-40 min, stirring occasionally. Add sugar & cook until deep golden.
Simmer: Stir in flour, then add wine (if using) & broth. Season with thyme, salt & pepper. Simmer 20 min.
Toast & Broil: Toast baguette slices, place on soup bowls, top with cheese, and broil until melted.
Serve: Enjoy hot, cheesy soup! Bon appétit!