INGREDIENTS
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4 small portabella mushroom caps, roughly 3-inches in diameter or trimmed to 3-inches in diameter
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Olive oil (about 2 tablespoons)
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Kosher salt and freshly ground pepper
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2 pounds fresh ground 80-percent lean chuck
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1 (8-ounce) piece pancetta, cut into 1/2-inch pieces
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4 Kaiser rolls, split
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Unsalted butter, softened (about 2 tablespoons)
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4 slices aged Swiss cheese