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To make the purée: Position a rack in the center of the oven and preheat the oven to 400ºF. Lightly butter a 9-inch glass pie plate (or a 9-inch ceramic quiche dish).
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Put the potatoes on a large baking sheet and roast until they are tender when pierced with a knife, about 50 minutes. Let them cool until easy to handle, about 15 minutes. (I roasted my potatoes a day in advance, then wrapped them in foil and stored them in the fridge until the next day.)
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Reduce the oven temperature to 350ºF (or preheat, if you've roasted the potatoes in advance).
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Peel the sweet potatoes and put the flesh in a medium bowl. Add the cream, brown sugar, 2 tablespoons of butter, lemon zest and juice, and nutmeg. Using a handheld electric mixer on medium speed, beat the mixture until smooth. Beat in the egg yolks. Spread the purée in the baking dish.
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Bake the purée until it is set and barely beginning to brown, about 30 minutes. (At this point the dish can be cooled, covered with foil, and refrigerated for up to 1 day. Reheat, covered, in a preheated 350ºF oven until heated through, about 20 minutes. Uncover the purée.)
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To make the meringue: Using the mixer with clean beaters, beat the egg whites in a medium bowl until soft peaks form. Gradually beat in the granulated sugar until the mixture forms stiff, shiny peaks. Using a spoon, spread and swirl the meringue over the purée.
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Bake until the meringue is touched with golden-brown, 7 to 10 minutes. Serve immediately.