INGREDIENTS
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3/4 lb. dried spaghetti
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3 Tbs. extra-virgin olive oil
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1/2 cup unsalted butter
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3 large portabella mushroom caps, gills scraped out and discarded, caps thinly sliced and cut into 2-inch pieces
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Kosher salt and freshly ground black pepper
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2/3 cup loosely packed fresh sage leaves
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1/3 cup toasted walnuts, coarsely chopped
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1/2 cup freshly grated Parmigiano Reggiano