- 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
For orange filling:
- Sprinkle gelatin over 1 tablespoon water in small bowl.
- Let stand 10 minutes to soften. Combine Sugar, 2 tablespoons orange peel, and lemon peel in medium metal bowl.
- Using fingertips, rub ingredients together until Sugar is moistened.
- Add eggs; whisk until smooth. Whisk in both juices.
- Place bowl over saucepan of simmering water (do not allow bottom of bowl to touch water);
- Whisk constantly until mixture thickens and thermometer inserted into mixture registers 180 °F, about 5 minutes. Remove from heat.
- Add gelatin mixture and stir until dissolved.
- Strain orange mixture into blender; let cool until thermometer inserted into mixture registers 140 °F, about 10 minutes.
- With blender running, add room-temperature butter 2 pieces at a time and process until blended.
- Continue blending 3 minutes longer.
- Transfer filling to bowl; mix in remaining 2 teaspoons orange peel.
- Cover and chill overnight.
- For crust:
- Butter 9-inch square tart pan with removable bottom.
- Finely grind flour, Sugar, nuts, and salt in processor.
- Add butter and cut in using on/off turns until mixture resembles coarse meal.
- Add egg yolk and blend in using on/off turns just until moist clumps form (do not allow dough to form ball). Press dough onto bottom and up sides of prepared pan (crust will be about 1/4 inch thick).
- Using thumb, press around sides to extend crust 1/4 inch above edge of pan. Chill 1 hour.
- Preheat oven to 375 °F. Bake crust until golden brown, about 20 minutes.
- Cool crust completely on rack. (Crust can be made 1 day ahead.
- Store airtight at room temperature.)
- Stir orange filling to loosen.
- Spoon enough chilled orange filling into crust to fill completely.
- Chill tart at least 1 hour and up to 4 hours.
- Garnish with candied blood orange slices.
- Drizzle oranges with some of syrup. Serve cold.