INGREDIENTS
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1 tablespoon unsalted butter
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7 or 8 pounds Spanish onions (about 10 to 15), thinly sliced
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Coarse salt
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Freshly ground black pepper
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6 to 12 slices baguette or any other country-style bread, preferably large enough to cover the width of your serving bowls
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1/3 cup sherry
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Red- or white-wine vinegar (optional)
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Red wine (optional)
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1/2 to 3/4 pound Gruyere or Emmenthaler cheese, grated