Instructions:
- Melt butter and oil in a large, heavy-bottomed Dutch oven over medium-low heat. Add the sliced onions and initial salt. Stir frequently for the first 15 minutes.
- Reduce heat to low. Continue cooking the onions slowly, stirring every 5–10 minutes, for 45–60 minutes, scraping up any brown bits (fond) from the bottom. The goal is a deep, dark mahogany colour.
- Sprinkle flour over the onions and cook, stirring constantly, for 2 minutes. Pour in the sherry/wine, scraping vigorously to lift all the browned bits from the bottom of the pan. Let the liquid reduce by half.
- Pour in the beef broth. Add thyme bundle and bay leaves. Bring to a gentle simmer, then cover partially and cook for 30 minutes to allow flavours to meld. Remove the thyme and bay leaves. Taste and adjust seasoning.
- While the soup simmers, lightly toast the baguette slices until pale gold.
- Preheat broiler. Ladle hot soup into oven-safe crocks, filling about 3/4 full. Float 1–2 slices of toasted bread on top of the soup.
- Generously cover the bread entirely with grated Gruyère cheese, ensuring the cheese touches the rim of the bowl.
- Place crocks on a baking sheet and place under the preheated broiler. Watch carefully (1–3 minutes) until the cheese is fully melted, bubbling vigorously, and deeply browned in spots. Serve immediately.