- 2 tablespoons extra-virgin olive oil
- 2 large onions, halved and thinly sliced (7 cups)
- 2 leeks, whites and light green parts only, thinly sliced
- 1 cup thinly sliced shallots (2 large)
- Kosher salt
- 5 garlic cloves, minced
- 3 tablespoons tomato paste
- 1 tablespoon herbs de Provence
- 1 bay leaf
- 1 cup dry red wine
- 1/4 cup sherry vinegar
- 8 cups beef stock or low-sodium broth
- Freshly ground pepper
- 1 stick unsalted butter, cut into chunks
- 12 baguette slices, cut 1/2 inch thick
- 4 ounces Gruyère cheese, shredded (1 cup)
- Finely chopped thyme, for garnish
- In a large enameled cast-iron casserole, heat the olive oil. Add the onions, leeks, shallots and a generous pinch of salt and cook over moderate heat, stirring occasionally, until very tender and caramelized, 40 to 45 minutes; stir in 1 to 2 tablespoons of water if the onions look very dry.
- Add the garlic, tomato paste, herbes de Provence and bay leaf to the casserole and cook over moderate heat, stirring, until fragrant and the mixture is glossy, about 3 minutes. Add the wine and vinegar and simmer over moderate heat, stirring, until reduced by half, 2 to 3 minutes. Stir in the stock and bring to a boil. Simmer over moderately low heat, stirring occasionally, until the soup is well flavored and reduced to 8 cups, about 45 minutes. Discard the bay leaf. Season the soup with salt and pepper and keep warm over very low heat.
- Meanwhile, in a very large skillet, melt the butter. Add the baguette slices and cook over moderate heat, turning once, until golden and just crisp, about 2 minutes. Transfer the croutons to a plate.
- Preheat the broiler and position the rack 8 inches from the heat. Arrange 6 heatproof bowls on a large rimmed baking sheet. Ladle the soup into the bowls and top with the croutons and cheese. Broil until the cheese is melted and browned in spots, 1 to 3 minutes. Sprinkle with thyme and serve.
The onion soup can be refrigerated for up to 2 days. Reheat gently before topping with croutons and cheese.