Pastry Shell
2 cups unbleached flour
2 tablespoons granulated sugar
1/2 teaspoon salt
1/2 cup plus 2 tablespoons butter, cut into bits and chilled
2 tablespoons solid vegetable shortening, chilled
1/4 cup ice water

Tart Filling
Zest of 3 lemons
1/2 cup fresh lemon juice
1/2 cup butter, softened
4 eggs
5 egg yolks
1 cup granulated sugar
1 1/2 teaspoons unflavored gelatine
1/4 cup cold water

Prepare Pastry Shell. Combine flour, sugar and salt in food processor. Whirl to blend. Drop in butter; pulse 3 or 4 times. Add the shortening and process until the mixture resembles coarse crumbs. With the machine running, add the ice water, 1 tablespoon at a time. When the mixture forms lumps, remove the dough from the work bowl and quickly press into a patty. Cover the dough with plastic wrap and chill for 30 minutes. Preheat oven to 400 degrees F.

Roll dough out into an 11-inch circle. Ease into a 10-inch tart pan with removable bottom. Trim the edge. Prick the bottom all over with a fork. Bake the shell for 15 to 20 minutes or until golden. Let cool.

Prepare Tart Filling. Combine lemon zest, lemon juice, butter, eggs, yolks and sugar in a heavy enamel saucepan. Cook the mixture over low heat, whisking constantly, until slightly thickened. Do not let boil. remove from heat. Sprinkle gelatine over the cold water in 1-cup glass measuring cup. Let stand 2 minutes for gelatine to soften. 

Place cup in small saucepan of simmering water, stirring gelatine occasionally to dissolve, about 2 minutes. stir gelatin into lemon mixture until well blended. Spread the filling evenly in the pastry shell. Chill completely before serving. Remove sides of tart pan. Decorate with citrus leaves.