Heat the butter in a heavy-bottomed pot or dutch oven (at least 5 quarts/litres) over medium heat. Add onions and a pinch of salt. Cook onions, stirring occasionally until they are deep golden brown, about 40 minutes.

Add white wine and cook, stirring, for 2 minutes. Add beef broth, water, thyme, bay leaf, pepper and a good pinch of salt (how much will depend on how salty your broth is - you can add more later). Bring to a boil then reduce heat to low, cover the pot and simmer for 30 minutes.

Meanwhile, preheat oven to 450ºF.  Arrange baguette slices on a baking sheet and toast 4 minutes on each side.

Ladle soup into oven-proof bowls or ramekins and top with a toasted baguette slice. Cover the toast with a slice of brie and broil until cheese is bubbly, just a minute or two, watching very closely.

(Alternatively, you can leave the toasted baguette on the baking sheet and arrange the brie on top then broil it that way. Once bubbly, transfer the cheese toasts to top individual soup portions.)