INGREDIENTS
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(½ stick) unsalted butter or duck fat
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large onions (about 5 lb.), thinly sliced
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large shallots, finely chopped
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garlic cloves, finely chopped
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Kosher salt and freshly ground black pepper
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dry vermouth or dry white wine
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white wine vinegar
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sprigs flat-leaf parsley
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sprigs thyme
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bay leaf
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low-sodium chicken or vegetable broth
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¼"-thick slices country-style bread, toasted
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sliced Comté cheese or Gruyère