Fresh Orange Tart with Hazelnut Crust

Name: Fresh Orange Tart with Hazelnut Crust
Ingredients: 1 1/4 teaspoons unflavored gelatin; 1 tablespoon water; 1 cup sugar; 2 tablespoons plus 2 teaspoons finely grated orange peel; 2 teaspoons finely grated lemon peel; 4 large eggs; 3 /4 cup fresh orange juice; 3 tablespoons fresh lemon juice; 1 cup plus 6 tablespoons (2 3/4 sticks) unsalted butter, cut into tablespoon-size pieces, room temperature; 1 1/4 cups all purpose flour; 1 /2 cup powdered sugar; 1 /4 cup hazelnuts; 1 /4 teaspoon salt; 9 tablespoons chilled unsalted butter, cut into 1/2-inch cubes; 1 large egg yolk; Candied Blood Orange Slices (click for recipe)