This is the soup you make when the house is cold and the onions have piled up. You cook them slowly, far longer than feels reasonable, until they turn mahogany and smell almost of caramel. Then a proper beef broth, a slice of stale bread, and a reckless blanket of Gruyère.
Paris served it at Les Halles in the small hours, when the market wanted something hot and cheap that tasted expensive. Ovenproof bowls are half the recipe: the broiler must catch the cheese until it bubbles and even blackens in spots.
Don't rush those onions. If they stay pale, the soup stays pale. Carry the bowls to the table still hissing. A Côtes du Rhône can stand up to all that cheese.
Method
- 01
Slice the onions as thinly as you reasonably can — they are the entire soup, so they deserve a little care. Melt the butter in a heavy pot, tumble them in, and cook over medium-low heat, stirring often, 40 to 50 minutes. They will weep, then turn gold, and finally a true mahogany. That last color is the flavor; if you rush them they scorch and the broth will taste of it.
- 02
Sprinkle the flour over the onions and cook, stirring, for 2 minutes — this little roux will give the broth a silk it would otherwise lack. Pour in the wine and let it bubble up, scraping the bottom so nothing of that mahogany is wasted. Add the stock, bouquet garni, cognac, salt and pepper, and simmer 20 minutes. Taste; the broth should be deep and winey, not thin.
- 03
Heat the broiler until it is fiercely hot. Toast the baguette slices until they are dry and golden; they will soak up the soup, so they need a little backbone. Discard the bouquet garni and ladle the soup into ovenproof bowls, filling them nearly to the rim. Work quickly; the bowls should go under the broiler roaring hot.
- 04
Float the toasted bread on the soup and blanket it generously with the Gruyère — don't be timid; the cheese is the lid. Set the bowls under the broiler until the cheese bubbles, browns, and even blackens in spots. Serve at once, while it is still hissing. A lukewarm onion soup is a wasted afternoon.
