INGREDIENTS
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4 russet potatoes, peeled
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1 cup shredded Gruyere
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1/2 cup grated Parmigiano-Reggiano
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1/2 cup grated Pecorino-Romano
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1 stick (8 tablespoons) unsalted butter
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2 tablespoons chopped fresh rosemary
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2 tablespoons chopped garlic
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1/2 teaspoon grated nutmeg
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1/2 cup panko breadcrumbs
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3 cups thinly sliced onions
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Kosher salt and freshly cracked black pepper
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1/2 cup heavy cream