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1
Make the crust: Pulse flour, cornmeal, sugar, sage, salt, and zest in a food processor until combined.
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2
Add butter; pulse just until mixture resembles coarse meal.
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3
Whisk together egg yolks and ice water in a small bowl.
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4
Drizzle egg mixture evenly over flour mixture, and pulse just until dough holds together.
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5
Divide dough in half, and shape each portion into a disk.
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6
Wrap in plastic, and refrigerate 1 hour or up to 2 days.
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7
On a lightly floured surface, roll out 1 disk of dough to an 11-inch round.
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8
Gently press into a 9-inch tart pan with a removable bottom; trim excess dough flush with rim.
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9
Refrigerate or freeze until firm, about 30 minutes.
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10
Reserve remaining dough for another use (it can be frozen up to 3 months; thaw in refrigerator before using).
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11
Preheat oven to 375F.
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12
Pierce bottom of shell all over with a fork.
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13
Bake until golden brown, about 25 minutes.
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14
Let cool.
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15
Make the filling: In a small bowl, sprinkle gelatin over the water, and let stand until softened, about 5 minutes.
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16
Prepare an ice-water bath.
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17
Whisk together egg yolks, sugar, and salt in a large heatproof bowl.
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18
Gradually whisk in lemon juice.
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19
Place bowl over (not in) a pan of simmering water, and whisk constantly until mixture has thickened.
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20
Whisk in softened gelatin.
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21
Remove from heat, and whisk in butter, a few pieces at a time, until smooth.
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22
Let cool, stirring occasionally.
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23
Place bowl with yolk mixture over ice bath, and stir until slightly thickened, about 2 minutes.
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24
Pour curd into crust; smooth top.
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25
Dollop small spoonfuls of creme fraiche all over top.
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26
Using a wooden skewer or the tip of a knife, swirl creme fraiche into curd to create a marbleized pattern.
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27
Refrigerate until set, 2 hours, or, wrapped in plastic, up to 1 day.
