The blood oranges give the panna cotta a gorgeous pink hue and a unique flavor, though regular oranges would work well too. Make the components a day in advance. In addition to freeing up a lot of time on the big day, the panna cotta and granita need plenty of time to set. The candied pistachios will keep for a long time, assuming you don't eat them all. They are that good.
FOR THE PANNA COTTA: Pour gelatin into a small bowl and moisten with 1 T cool water. Let bloom for 1 or 2 minutes. In a small saucepan, combine half of the blood orange juice with the sugar on medium heat, and bring just to a simmer. Add gelatin and the finely-grated zest. Remove from heat when gelatin fully dissolved. Set aside.
In a medium bowl, combine remaining orange juice with creme fraiche and buttermilk. Strain hot juice-gelatin liquid into the bowl, and whisk gently to mix evenly. Portion into ramekins. Refrigerate and allow to set, about 4 to 6 hours, or overnight.
FOR THE GRANITE: Pour orange juice into a large mixing bowl. Combine lemongrass, zest strips, sugar, and water in a saucepot and bring to boil. Remove from heat and strain into the orange juice. Stir. Pour into a large freezer-safe storage container, cover and refrigerate. When cool, freeze overnight. When solid, scrape with a fork into ice flakes. Serve or store in the freezer in a sealed container.
FOR THE PISTACHIOS: On a sheet pan, toss pistachios in just enough confectioners' sugar to lightly coat. Combine water, sugar, and salt in heavy-bottomed skillet on high heat. Cook until sugar melts and lightly caramelizes to light brown (about 140 Celsius). Add the pistachios and mix vigorously to coat every nut with sugar mixture. Keep cooking and stirring until the nuts caramelize. Meanwhile, line the sheet pan with parchment or a Silpat. Spread nuts on the Silpat or parchment-lined sheet tray to cool.
ORANGE SUPREMES: With a sharp knife, cut the top and bottom off each orange. Trim rind and skin from sides, following the contour of the orange. Cut orange into segments by making a vertical cut on either side of each segment's white stripe. To serve, place 3 to 4 supreme slices on top of each panna cotta ramekin. Scrape 1 to 2 T of granite ice flakes from the block with a fork and add over supremes. Garnish with sea salt and a few caramelized pistachios.