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1
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.
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2
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 180u00b0F, 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 140u00b0F, about 10 minutes.
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3
With blender running, add room-temperature butter 2 pieces at a time and process until blended. Continue blending 3 minutes longer.
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4
Transfer filling to bowl; mix in remaining 2 teaspoons orange peel. Cover and chill overnight.
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5
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.
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6
Preheat oven to 375u00b0F. 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.)
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7
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.
