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Place one oven rack in the lower third of the oven and preheat the oven to 350°F.
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The lemons will be your serving dishes so prepare them as follows: trim the stem end from a lemon so that the fruit sits level.
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Cut the top end one-third of the way down, making sure the cut is parallel with the bottom; reserve the top. Repeat with all the lemons.
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Hold a lemon above a strainer set over a heat-proof bowl, and scoop out the pulp.
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Squeeze and reserve the juice; discard the pulp. Repeat with all the lemons.
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In a bowl of an electric mixer fitted with the whisk attachment, combine the egg yolks, 1/4 granulated sugar, 1/4 cup reserved lemon juice, and the flour.
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Beat on medium speed until the mixture is pale yellow, about 3 minutes.
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Fill a small saucepan one-quarter full of water and heat it to a simmer over medium heat.
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Place the bowl over the simmering water, and whisk constantly until very thick, about 8 minutes.
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Remove the bowl from the heat, and return to the mixer.
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Beat on medium speed until cool, scraping down the sides several times with a rubber spatula, about 10 minutes.
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Transfer to a medium bowl, and set aside. Wash and dry the mixer bowl and attachment.
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Combine the egg whites and the remaining granulated sugar in the mixer bowl. Set a small saucepan filled with water over medium heat, and bring to a simmer.
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Place the bowl over the simmering water, and whisk until the sugar has dissolved and the mixture is warm.
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Remove from heat and return to the mixer fitted with the whisk attachments; beat on low speed until frothy.
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Gradually increase the speed until the meringue is shiny and soft peaks form, 2 to 3 minutes; do not overbeat.
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Whisk one-third of the meringue into the yolk mixture to lighten it, and whisk until smooth.
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Gently fold in the remaining meringue. Carefully fill the prepared lemon shells to just the rims.
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Transfer shells to a parchment-lined baking sheet.
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Bake until the meringue is slightly golden and rises about 1 inch above the lemon, about 15 minutes.
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Remove from the oven, and transfer to serving plates.
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Dust with the confectioner's sugar, and garnish with the reserved lemon tops.
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Serve immediately.