-
1
Heat butter and oil in a heavy-bottomed pot over medium heat; add onions and sugar, cook uncovered 35–40 minutes, stirring occasionally, until deep golden and caramelised.
-
2
Add wine and deglaze, scraping browned bits; simmer 5 minutes to reduce by half.
-
3
Pour in stock, add bay leaves and thyme, bring to simmer, and cook 15 minutes; season carefully with salt and Tellicherry pepper.
-
4
Discard bay leaves and thyme; divide soup among four ovenproof bowls.
-
5
Top each bowl with a toasted baguette slice; pile Gruyère AOP over bread.
-
6
Place under a preheated grill 3–4 minutes until cheese melts and bubbles; serve immediately.