French Onion Soup
Melt butter with olive oil in a large heavy pot over medium heat. Add onions and 1 tsp salt; cook stirring every 5 minutes for 55–65 minutes until deeply caramelised and jammy.
Deglaze with cognac, then white wine; scrape up all browned bits. Add thyme, bay leaf, and beef stock; simmer 20 minutes. Season with salt and pepper; discard herbs.
Ladle soup into oven-safe bowls set on a baking sheet. Float 2 toasted baguette slices on each, and pile with Gruyère.
Grill/broil until cheese is bubbling and spotted golden, 3–4 minutes. Serve immediately with caution — bowls will be extremely hot.