Photo curtesy of Williams Sonoma.
2 lbs. sweet potatoes1 tablespoon plus 2 tablespoons unsalted butter1/4 cup plus 1/4 cup grated Parmesan cheese1 medium white onion, minced3 tablespoons all-purpose flour1 1/2 cups milk1/4 teaspoon freshly grated nutmeg1/4 teaspoon allspice powder1/4 teaspoon ginger powder6 large eggs, separated1 1/2 cups coarsely grated Gruyere, 4 ounces
Preheat an oven to 375°F.
Pierce the potatoes two or three times with a fork, place on a baking sheet and bake until they can be easily pierced with a knife, 30 to 40 minutes. Reserve and cool. Cut the potatoes in half and scoop out the pulp and place in a bowl. Mash with a potato masher to make a smooth puree. You should have 2 1/2 cups of puree.
Using 1 tablespoon butter, butter a 2-quart souffle dish. Dust with 1/4 cup Parmesan cheese.
In a large saucepan, melt the remaining 3 tablespoons butter over medium heat. Add the onions and cook until soft, 10 minutes. Add the flour and cook, stirring constantly for 3 minutes. Add the milk, and simmer, whisking until the mixture has thickened, 2 to 3 minutes. Remove the pan from the heat and the nutmeg, allspice, and ginger and stir together. Add the egg yolks, one at a time, stirring well after each addition. Add the sweet potato puree and the Gruyere and stir together until well blended. Season to taste with salt and pepper.
In a bowl with an electric mixer, beat the egg whites until they just hold stiff peaks. Fold one-quarter of the egg whites into the sweet potato mixture to lighten it. Fold the remaining whites into the mixture gently yet thoroughly. Pour the mixture into the prepared souffle dish. Sprinkle the top with the remaining Parmesan cheese and bake in the middle of the oven until it is puffed and golden, 45 to 50 minutes. Remove from the oven and serve immediately.
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