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2 tablespoons unsalted butter |
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2 large sweet onions, halved and thinly sliced |
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3 tablespoons red wine vinegar |
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Salt and freshly ground black pepper |
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1/2 cup all-purpose flour |
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1/4 teaspoon fine sea salt |
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1/8 teaspoon freshly ground black pepper |
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Melted unsalted butter, for pan |
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8 ounces grated aged Gruyere cheese |
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12 to 16 slices thinly sliced black forest ham |
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