INGREDIENTS
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Coarse salt and freshly ground pepper
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3 pounds russet potatoes, peeled and cut into 1 1/2-inch pieces (8 cups)
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1/2 cup extra-virgin olive oil
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1/2 stick unsalted butter, cut into small pieces
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1 yellow onion, finely diced (1 cup)
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Garnish: 1 to 2 rosemary sprigs