1
Melt butter in a heavy pot over low heat. Add sliced onions and a pinch of salt.
2
Caramelise onions slowly for 45 minutes, stirring occasionally, until deep golden brown.
3
Deglaze with white wine and scrape up any browned bits.
4
Add beef stock and thyme; simmer for 15 minutes. Season to taste.
5
Ladle into oven-safe bowls. Top with a toasted baguette slice and a generous mound of Gruyère.
6
Broil until cheese is bubbling and golden.