-
1
In a large enameled cast-iron casserole, heat the olive oil.
-
2
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.
-
3
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.
-
4
Add the wine and vinegar and simmer over moderate heat, stirring, until reduced by half, 2 to 3 minutes.
-
5
Stir in the stock and bring to a boil.
-
6
Simmer over moderately low heat, stirring occasionally, until the soup is well flavored and reduced to 8 cups, about 45 minutes.
-
7
Discard the bay leaf.
-
8
Season the soup with salt and pepper and keep warm over very low heat.
-
9
Meanwhile, in a very large skillet, melt the butter.
-
10
Add the baguette slices and cook over moderate heat, turning once, until golden and just crisp, about 2 minutes.
-
11
Transfer the croutons to a plate.
-
12
Preheat the broiler and position the rack 8 inches from the heat.
-
13
Arrange 6 heatproof bowls on a large rimmed baking sheet.
-
14
Ladle the soup into the bowls and top with the croutons and cheese.
-
15
Broil until the cheese is melted and browned in spots, 1 to 3 minutes.
-
16
Sprinkle with thyme and serve.
