INGREDIENTS
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4 tablespoons (1/2stick) unsalted butter or duck fat
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6 large onions (about 5 lb.), thinly sliced
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2 large shallots, finely chopped
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2 garlic cloves, finely chopped
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Kosher salt and freshly ground black pepper
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1/2 cup dry vermouth or dry white wine
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1 tablespoon white wine vinegar
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4 sprigs flat-leaf parsley
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2 sprigs thyme
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1 bay leaf
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8 cups low-sodium chicken or vegetable broth
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8 slices 1/4-thick slices country-style bread, toasted
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8 ounces sliced Comté cheese or Gruyère