Instructions:
- Thinly slice all onions evenly. Ensure they are uniform so they cook at the same rate.
- Melt butter and oil in a heavy-bottomed Dutch oven over medium heat. Add all sliced onions, salt, pepper, and sugar. Cook, stirring frequently, for 15-20 minutes until softened and translucent.
- Reduce heat to medium-low. Continue cooking, stirring every 5-8 minutes, for an additional 30–40 minutes until the onions achieve a deep, mahogany brown colour. Scrape up any brown bits (fond) from the bottom as you go.
- Increase heat to medium. Pour in the sherry/wine. Scrape the bottom vigorously to lift all the caramelised bits. Let the liquid reduce by half (about 3 minutes).
- Sprinkle the flour over the onions and stir well for 1 minute. Slowly whisk in the beef stock. Add thyme and bay leaf. Bring to a simmer, then reduce heat and cook gently, partially covered, for 30 minutes.
- Remove herbs. Taste and adjust salt/pepper. Optionally, strain the soup base through a fine-mesh sieve for a smoother finish, then return the liquid to the pot.
- While the soup simmers, toast the baguette slices lightly in the oven or a dry pan until crisp.
- Preheat oven broiler. Ladle hot soup into oven-safe crocks, filling about 3/4 full. Float 1-2 toasted baguette slices on top of the liquid.
- Heap a generous amount of grated Gruyère (and Swiss/Provolone, if using) over the bread, ensuring some cheese drapes over the edges of the crock.
- Place crocks on a baking sheet and place under the broiler. Broil for 2–4 minutes until the cheese is bubbling, melted, and beautifully browned in spots. Serve immediately, warning diners the bowls are extremely hot.