Instructions:
- If time allows, lightly sauté the sliced onions in butter and oil in a skillet for 10-15 minutes until softened. If skipping, place raw onions directly into the slow cooker.
- Combine the onions, butter, and olive oil in the slow cooker insert. Cook on LOW for 8-10 hours or HIGH for 4-5 hours until the onions are deep mahogany brown and significantly reduced. Stir once or twice during cooking.
- Once onions are caramelized, stir in the sherry/wine (if using) and let it bubble for 2 minutes. Whisk in the flour until fully incorporated.
- Pour in the beef broth. Add the thyme and bay leaf. Season lightly with salt and pepper. Cover and cook on LOW for an additional 1 hour to allow flavours to meld.
- While the soup simmers, lightly toast the baguette slices until dry and golden.
- Remove the bay leaf. Ladle the soup into oven-safe crocks. Float 1-2 toasted baguette slices on top of the soup in each bowl.
- Generously top the bread with the grated Gruyère and a dusting of Parmesan.
- Place the filled crocks onto a baking sheet. Broil on high until the cheese is bubbly, melted, and beautifully browned (about 2-4 minutes). Watch very carefully to avoid burning! Serve immediately.