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1
Whisk yolks and eggs in medium bowl to blend. Combine lemon juice, butter, and 1/2 cup sugar in large metal bowl. Set bowl over saucepan of simmering water; whisk until butter melts and sugar dissolves. Gradually whisk 1/3 of hot butter mixture into eggs, then whisk mixture back into bowl with remaining butter mixture set over simmering water. Whisk constantly until custard thickens and thermometerinserted into mixture registers 180u00b0F, about 5 minutes (do not boil). Pour lemon curd through strainer set over medium bowl. Place plastic wrap directly on surface of lemon curd; chill overnight. (Can be made 2 days ahead. Keep refrigerated.)
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2
Finely grind hazelnuts in processor. Using electric mixer, beat butter, powdered sugar, and lemon peel in medium bowl until well blended. Add yolk and beat until smooth. Beat in hazelnuts, then flour, and salt. Gather dough into ball; flatten into disk. Wrap in plastic and chill 1 hour.
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3
Preheat oven to 350u00b0F. Roll dough out on floured surface to 12-inch round. Transfer to 9-inch-diameter tart pan with removable bottom; press crust onto bottom and up sides of pan; trim excess dough overhang. Chill 20 minutes. Bake crust until golden brown and cooked through, pressing with fork if c rust bubbles, about 25 minutes. Cool crust in pan on rack. (Can be prepared 1 day ahead. Cover and store at room temperature.)
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4
Using electric mixer, beat whipping cream, creme fraiche, 3 tablespoons sugar, and lemon peel in large bowl until stiff peaks form.
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5
Spoon half of lemon curd and half of lemon creme fraiche into another large bowl. Using small rubber spatula, gently fold curd and lemon creme fraiche together, creating marble effect and being careful not to overmix to retain marbling. Spoon marbled mixture in dollops into cooled crust. Using tip of knife or small rubber spatula, swirl and pull mixture upwards in peaks. Repeat with remaining curd and creme fraiche mixture. Refrigerate tart at least 1 hour and up to 4 hours.
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6
Remove pan sides. Place tart on platter. Serve with rhubarb compote.
