INGREDIENTS
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2 tablespoons extra-virgin olive oil
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2 large onions, halved and thinly sliced (7 cups)
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2 leeks, whites and light green parts only, thinly sliced
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1 cup thinly sliced shallots (2 large)
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Kosher salt
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5 garlic cloves, minced
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3 tablespoons tomato paste
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1 tablespoon herbs de Provence
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1 bay leaf
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1 cup dry red wine
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1/4 cup sherry vinegar
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8 cups beef stock or low-sodium broth
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Freshly ground pepper
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1 stick unsalted butter, cut into chunks
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12 baguette slices, cut 1/2 inch thick
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4 ounces Gruyère cheese, shredded (1 cup)
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Finely chopped thyme, for garnish