Soupe à l'Oignon Gratinée

Melt the butter in a wide, heavy-bottomed pot over medium-low heat. Add all the onions with a generous pinch of salt and stir to coat. Cook uncovered, stirring every 5–8 minutes, for 45–60 minutes total. Do not rush this: the onions must pass through transparent, then blond, then golden, then a deep mahogany-brown. They should reduce to roughly one-fifth of their original volume and smell sweet and nutty, never scorched.

Pour in the white wine and scrape vigorously with a wooden spoon to lift all the browned fond from the pot bottom. Add the thyme sprigs. Let the wine reduce until almost completely absorbed, about 3–4 minutes — you should hear an aggressive sizzle that fades to a quiet bubble.

Pour in the beef stock and bring to a gentle simmer. Cook uncovered for 20 minutes, allowing the onion sweetness and stock depth to meld. Taste and correct seasoning with salt and pepper. Fish out the thyme stems.

Ladle the hot soup into oven-safe crocks placed on a baking sheet. Float one or two toasted baguette slices on each surface. Pile the Gruyère generously over the bread and the rim — the cheese should completely cover the surface and drape over the edge of each crock. Broil on the top rack for 4–6 minutes until the cheese is deeply golden, bubbling, and spotted with brown blisters.