Instructions:
- Slice all onions thinly and uniformly. Have the butter and oil ready in a large, heavy-bottomed Dutch oven or stockpot.
- Melt butter and oil in the Dutch oven over medium heat. Add onions, stir well to coat. Cook uncovered for 15 minutes until softened and translucent.
- Reduce heat to medium-low. Cook the onions slowly, stirring every 5-10 minutes initially, then every 3-5 minutes as they darken. Continue this stage for 45–60 minutes until the onions are a deep, mahogany brown.
- Push onions to one side. Add flour and cook for 1 minute. Deglaze the pot by pouring in the sherry/wine, scraping up any browned bits (fond) from the bottom. Let the liquid reduce slightly (about 2 minutes).
- Pour in the beef broth. Add thyme and bay leaf. Bring to a gentle simmer, reduce heat to low, cover partially, and cook for 20 minutes to let the flavours meld. Remove thyme stems and bay leaf. Taste and adjust salt/pepper.
- While the soup simmers, lightly toast the baguette slices under the broiler or in a 350°F (175°C) oven until dry and pale golden.
- Preheat the broiler. Ladle hot soup into oven-safe crocks. Float 1-2 toasted baguette slices on top of the soup.
- Generously cover the bread with grated Gruyère cheese, ensuring edges are covered. Place crocks on a baking sheet and broil for 2–4 minutes until the cheese is bubbling, melted, and beautifully browned.
- Carefully remove from the oven and serve immediately, warning diners that the crocks are extremely hot.