1/4 teaspoon unflavored gelatin
1 tablespoon cold water
6 large egg yolks
1 cup sugar
1/4 teaspoon salt
1/2 cup fresh lemon juice
1/2 cup (1 stick) unsalted butter, cut into tablespoons
3 tablespoons creme fraiche
Makes enough for two 9-inch tarts
2 1/4 cups all-purpose flour, plus more for work surface
3/4 cup coarse yellow cornmeal
3 tablespoons sugar
1 tablespoon plus 1 teaspoon finely chopped fresh sage
1 1/2 teaspoons salt
1/2 teaspoon finely grated lemon zest
3/4 cup (1 1/2 sticks) cold unsalted butter, cut into small pieces
3 large egg yolks
5 tablespoons ice water
1. Sprinkle gelatin over cold water in a small bowl, and let stand until softened, about 5 minutes.
2. Whisk together egg yolks, sugar, and salt in a large heatproof bowl. Gradually whisk in lemon juice. Place bowl over a saucepan of simmering water, and whisk constantly until mixture has thickened and registers 160 degrees on an instant-read thermometer. Whisk in gelatin mixture. Remove from heat, and whisk in butter, a few pieces at a time, until smooth. Let cool, stirring occasionally. Prepare an ice-water bath. Place bowl of yolk mixture over bath, and stir until slightly thickened, about 2 minutes.
3. Spread curd into crust; smooth top. Dollop creme fraiche on top. Using a wooden skewer or the tip of a knife, swirl creme fraiche into curd to create a marbleized effect. Refrigerate until firm, at least 2 hours (or up to overnight).
1. Pulse flour, cornmeal, sugar, sage, salt, and lemon zest in a food processor until combined. Add butter; pulse until mixture resembles coarse meal. Whisk egg yolks and ice water in a small bowl. With machine running, add to flour mixture through feed tube; process until dough just holds together.
2. Turn out dough onto a work surface. Divide in half, and shape each portion into a disk. Wrap in plastic, and refrigerate 30 minutes (or up to 2 days).
3. On a lightly floured work surface, roll out 1 disk to a 10-inch round. Fit into a 9-inch tart pan with a removable bottom; trim edges flush with rim. Refrigerate until firm, at least 1 hour (or up to 1 day). Reserve remaining dough for another use (it can be frozen up to 3 months).
4. Preheat oven to 375 degrees. Prick bottom of tart shell with a fork. Bake until golden brown, about 25 minutes. Let cool.