01 -
Heat olive oil and 2 tablespoons butter in a large heavy pot over medium heat until butter melts. Add onions, stir to coat, cover and cook for 20 minutes, stirring every few minutes until transparent.
02 -
Remove lid, add sugar and salt. Continue cooking uncovered for 40 minutes, stirring occasionally, until onions are deep golden and sweet.
03 -
Stir in all-purpose flour and remaining 1 tablespoon butter. Cook for 2 to 3 minutes to eliminate raw flour taste.
04 -
Pour in 1 cup beef broth and stir vigorously to scrape up browned bits from the bottom. Cook for 1 minute allowing liquid to evaporate slightly.
05 -
Add remaining beef broth, red wine, bay leaves, and thyme sprigs to the pot.
06 -
Bring to a boil, reduce heat, and simmer uncovered for 30 minutes. Remove bay leaves and thyme sprigs.
07 -
Adjust salt and pepper to taste. Stir in cognac.
08 -
Lightly brush baguette slices with olive oil and toast under broiler 30 seconds to 1 minute per side until golden.
09 -
Top soup with 1 cup Gruyère cheese, place toasted bread slices over cheese, then layer remaining Gruyère and Parmesan cheese. Bake in preheated oven at 350°F (177°C) for 20-25 minutes until cheese is melted and golden.